Which amp will cause a larger increase in electric bill?

Threshold SA/4e or Threshold S500 II?  How to determine?  I cant find idle draw on SA/4e.

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In general, if it is solid state, I would not worry about power consumption or your electric bill.   It doesn't matter if it is Class D or Class AB.   Granted Class A amps do consume some power when it is on and idling but most Class A amplifiers are not pure Class A over their entire power range.  If you do spring for pure Class A, do ask the manufacturer what the idle current is like, as they have to deal with that power in a big, expensive way.  These days, I suspect even the pure Class A guys turn their bias down during long idle periods. 

If it is a vacuum tube amp, then yeah, you will definitely see it on your bill.  Big time.   In a big way.   Trust me on this.  If you doubt me, just ask my checkbook!  My system burns up about 1000 Watts when idling, so I turn it off when not in use. 

There are some new regulations on idle power consumption when the equipment is plugged in but not in use.   I believe the power level is under 1W now, about a half Watt or so;  I know it has been dropping over the last decade.  

That being said, I would not leave your equipment on 24/7 unless you make a point of playing it every single day.   In that case, perhaps leaving it on would make more sense.   Although for vacuum tubes, that may not be the case.   Some tubes have finite life times regardless if they are playing music or not.  Simply being on is burning up lifespan.  Again, check with the manufacturer. 
I don't believe those are close to current Class A amps and they probably do draw a lot of current on idle.  They are the least efficient of all amps.  I had an Odyssey Stratos extreme class AB amp for 5 years which cost about 30.00 a month to run as they are designed to be on all the time.  My electric bill went down that amount after selling it and going with something else.
Class A amps draw approximately 5 times their rated power at idle (and at load). The 100 wpc SA/4e will probably draw about 1,000 watts or a kilowatt hour of electricity when it’s powered on.

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A) Only single ended class A is 20% efficient. Push-pull class A is about twice as efficient. 

B) That thing is nowhere near big enough to dissipate even 400 watts of heat. 
My 100W class A Levinson 20.6 mono draws 500 watts at idle (4 amps measured at the breaker). That’s 20% efficient and it is most definitely a push-pull, not single-ended.

The 100 watt Pass XA100.8 mono draws 420 watts at idle. I am pretty sure that one is a push-pull also.

Push-pull amps have UP TO a 50% efficiency. I know for a fact the Levinson has a push-pull Darlington in the driver stage to keep a surge of bias current flowing for the output stage, which adds to the inefficiency.
Actually, if an amp is pure Class A, it won't matter if it is push pull or not, assuming the power output is the same between the two.  

The reason is the maximum load current flows 100% of the time, so if the power output is the same, then the dissipated power at idle is the same.   What push pull does buy the designer, is it puts the dissipation across two complimentary devices stacked in tandem, which makes heatsink design easier.  However, the end buyer still pays the same electric bill. 

A number of amplifier companies define their products as Class A, when in reality they are Class A at lower power levels and Class AB at higher power levels.   I do note now that some amp companies state where their amps are Class A and where they become Class AB.   I like honesty in companies.

So, if you understand everything I just wrote, then realize that the only design difference between Class A and Class AB is the bias current and the impact that has on the heatsink and power supply.  
BTW, just to clarify, when I state Class A, I mean the entire amplifier is Class A.  There are quite a number of amplifiers that have Class AB output stages with Class A driver and amplification stages.   Those amps are still Class AB by IEEE definition.  
Thanks for responses.  So let me clarify.  If sitting idle (power on), which will cost more, a class A 100 watt (Threshold) vs a 250 watt A/AB (Threshold)?  I have both and a few more.  I'm not worried about electric bill, just wondering.
The only way to correctly answer your question is to find out the RMS current of the two amplifiers at idle and compare them.    This is how you do it -> 

The wattage consumed for each will be the respective current times 120.   Divide that power by 1000 to get Kilo Watt Hours, KWH.  Then, do two things:  First, multiply the KWH number by the number of hours the amp will operate.   If you are leaving it on 24/7, then multiply the KWH by 720 (30x24) to get KWH for the monthly billing.  The second thing you do is look up the "Maximum" KWH charge on your electric bill.    Multiply the KWH number for the amplifier by that max KWH charge.  That will give you the $ charge per KWH on your electric bill for that amplifier.   Note that some utility companies charge extra if you consume too much power, so that number might bounce into the next charge bracket.  If so, then the charge will be a bit more. 

For example, assume one amp draws 100W at idle.   So, 100/1000 = 0.1 KWH.  If you leave it on 24/7, then you will use 0.1 x 720 = 72 KWH.  If your max charge on your electric bill is 30 cents per KWH, then you will be charged $21.60 to run that amplifier, $0.3 x 72KWH.  

Another way to look at it is if one amp pulls 100 W and another pulls 120W, then the 120W amp will cost you 20% more money to run it than the 100W unit.   But if you want to know what that dollar number is, you have to do the math. 

Lastly, this will be a very close approximation.   The reason is we assume the line voltage is 120VAC, which it won't always be that number.   It will vary, both up and down.   Consequently, the charge will vary a bit.   Also, that 120VAC is nominal for North America, it is different for other countries, that is 100VAC for Japan, 230VAC for Australia, etc.   Use the nominal line voltage for your country and make sure that idle current was measured at the line voltage you are going to use. 
If sitting idle (power on), which will cost more, a class A 100 watt (Threshold) vs a 250 watt A/AB (Threshold)? I have both and a few more. I'm not worried about electric bill, just wondering.

Kill A Watt
Huh... and here I am looking like the dumbass with a 2x 31wpc class A amp that's 34% efficient. Build Pass I guess....
The Kill A Watt meter is ideal for things like freezers or refrigerators.  These items vary their power consumption on whether their compressor is running, how often the door is opened, and what the ambient temperature happens to be.   For amplifiers in idle, just measure the static idle current.   Of course, this assumes you have an ammeter handy.    For the 15$ it is cheaper to get the KAW meter than to buy a decent DMM.
imhififan - Well, if the Class A Threshold is really Class A, that will consume a lot more power.   When it is on and thermally stable, you should feel a bunch of heat rising out of it as it is dissipating around 500W or so just sitting there. The Class AB amp is a lot less - I'm guessing maybe 50 W or so.