A watt is a watt is a watt. It costs a lot more to build a pure class A amp than a class AB amp of the same power. Class A amps are very inefficient, need large transformers (power and output, if applicable), and dissipate a lot of heat.
For all their disadvantages, Class A amps often have a kind of liquidity that is hard to find in Class AB designs.
Class A amps go into clipping more gracefully than class AB amps. This sometimes leads users to push the class A amp hard into a load because of the gradual move into distortion (and largely 2nd harmonic distortion at first, which is pleasing to the ear).
So the L509u is capable of much more power than the L590Aii. There is one more factor to consider, though, and that is how the two amps behave as the load impedance changes. Speaker impedance often varies across the frequency spectrum, and it's not clear how well the two amps deliver current to lower impedances. Class A amps often but not always have an edge here due to their large power supplies and basic topology.
If you are using efficient speakers consider the L590Aii. It will emphasize finesse over brute force. 
An amplifier is a tool to drive your speakers, select them first and the select an appropriate one that you like the sound of on THAT speaker. There are many factors involved, ghostrider has pointed several of them out. Generalizations about the qualities of Class A, Class B, Tubes etc. are less important than the qualities of a particular amp and how well it matches your speaker. 
Big watts.. little tiny watts. Tubes have BIG watts... Class A amps usually have BIG watts because they have big power supplies.. Cheap class AB amps have little tiny watts, unless they are BIG, really BIG (and expensive).. and can double the wattage with halving the impedence. (Think Bryston 28BSST*) But really a Watt is a Watt. (though some small amps cheat and claim they have more: that is the 'little' watt side of the story. Really a 50 watt amp only puts out 50 watts into 8 ohms, and a 200 watt amp puts out 200 watts into 8 ohms. BUT!! that 50 watt amp may put out 100 watts into 4 ohms and 200 watts into 2 ohms.. (a few such heroic amps can do 400 watts into 1 ohm) but the 200 watt into 8 ohm amp may only put out 250 watts into 4 ohms and actually fizzle to less than 100 watts into 2 ohms, and explode (blow the fuses) when it sees a 1 ohm load. This is only to confuse you as the power handling ind low imepdence loads has NOTHING to do with class "A" or class AB However it all fits into the big picture of how power amps are rated. 
The other point is that many AB amps operate in class A until a certain wattage is reached, then go AB.
I used to have a 75 watt (into 8 ohms) class A amp that was beautiful to listen to. 
Excellent answers, I always wondered about this too.
Thanks. 
I owned an Aleph 30 (Class A) amp rated at 30 glorious watts per channel. Based on the different amps I have owned previously and a direct comparison to them, it had no more power than just a 30 watt per channel AB solid state amp. 
Many thanks for all the helpful replies! Now I am less confused.
The speakers that I am considering are the Verity Audio Parsifal Ovations, driven by either a Luxman 590Aii or a 509u. Both Luxman integrateds are heavy beasts with large transformers/power supplies, and they exactly double their output into 4 ohms. One is class A (590Aii, 30 wpc) and the other class AB (509u, 120 wpc).
The 509u probably operates in class A at low volumes, and may be the way to go. I know that the 590Aii is highly regarded, but I am concerned about a hot and power hungry pure class amp in the longterm.
Adding a Wadia 381 or an Esoteric X05 cdp to the mix may be the beginning of a great system.
Regards,
Mark 
Mark I think the deciding factor should be your ears;if you can bring the 2 Luxman units home with the speakers to see how they interact in your listening room;let them warm up for a set period of time then sit down and enjoy the decision making process.Hopefully you can do a home audition of the pieces in question;good luck and enjoy the listening. 
Mark  When you listen to 200W peaks your class AB amp really delivers to speakers on average only few percent of peak power. It is because music is not always at the peak and half as loud means 1/10 of power. In addition there are gaps (unless you listen to sinewaves). Nobody sane would design power supply in 2x200W amp for 800W required (50% efficiency at best). Typical amp can deliver full power at limited period of time (like 1 minute) before thermal protection kicks in. There is no need to design large heatsinks for the same reason  otherwise they would have to be the same size as in class A amps.
Class A is extremely wasteful but has one big advantage  is more linear before negative feedback is applied. At the end (with feedback) both class A and class AB might have the same linearity but class AB required 20x more gain (before feedback) to achieve this. This huge gain (in order of 4000) in connection with limited response of the amp (limited slew rate) is causing late feedback correction. In time domain it shows as small overshoots of pulses and in frequency domain it shows as enhanced higher order odd harmonics (bright sound). This is called TIM (Transient Intermodulation Distortion) and was unknown until 70's. There are great class AB amps where designer minimized the problem and also some bad class A amplifiers. You have to listen. My general intuition is that better specifications mean worst sound (achieved by deeper feedback). Same with power  what is on the paper is useless. The only usefull parameter you can find there is weight (some correlation with power). 
I noticed now "Many thanks for all the helpful replies! Now I am less confused"
Mark, I hope you don't feel more confused after my post (I tend to complicate things). 
One other thought here is this... listening levels.
Listening levels are achieved only by the input of power to the speakers... ergo, watts. Class A watts, Class D watts or Class A/B watts... it'll take a certain amount of them to produce a certain SPL.
AS often as I've come over this issue, I continually forget the where abouts of the conversion chart online.
The amount of watts needed to produce the level you desire at times going into your speakers is quite important! An insufficient amount can put them in harms way either immediately or over some period of time if they are in fact being under driven, or starved for power, when trying to achieve the levels you wish now and then.
80% of the music is made in the first 20 watts.
But if the Verity's aren't very eff, or have some severe impedance drops with some freqs, more power is surely going to be needed than those first 20 watts.
I'm no paint peeling listener myself usually. 8590% of the time I'm under 90db SPL. But there are those times one may wish to twirl the fun knob up.
Don't risk downtime or addded expense by runnnig with insufficient power.
Just a thought no one had brought up yet.
Have fun.

Just an after thought, AB also runs a lot cooler than class A amps. 
In general, I love tube amps. I know this is a generalisation, but the solid state amps I have found listenable to, were nearly all Class A, the Aleph 3 being the best. I would love to hear the new integrated based on the XA30, I expect that will be quite special. Otherwise, I found an edge, lack of musicality, harshness, call it what you will, to class A/B, SS amps. The only exception was the Lavardin IT, which was very nice indeed. 
Mark Are you buying new?Who carries Luxman in Toronto?I got a ML 27.5 and yes it's class AB but boy does this amp ever sing! Regards George 
Yes, I plan to buy new. My Kind of Music, near Heath & Spadina in Toronto is a real treasure trove... http://www.mykindofmusic.com/index2.htm
John carries Luxman and Verity Audio, along with many other lines.
Regards,
Mark 
An AB amp that is class A to X watts is not equivalent to a class A amp, bear in mind  the distortion spectra are different even within the class A range (and that is largely what defines the sound of an amp). Nelson Pass has an excellent article explaining this on his FirstWatt site.
Furthermore, only a full class A amp can be singleended as well (as the Pass Alephs are)  that change in topology seems to make an even bigger difference to the sound.
There sure are some greatsounding class AB (SE and PP) amps these days, tho! 
Speaking of SS amps....Does anyone know the class A bias for Ayre MXr amp? 