High power rating=better speakers?

As above -- indeed, some of the best drive units are not garanteed to withstand even 50W continuous.
BTW, do keep in mind that drive units are the ultimate limitation and most (including the ones your spkrs are wearing) won't take 100W energy let alone 500 -- for more than 1-2 msec. So, please don;t blast them!

My experience is that this is all very deceptive. I have found 85 db sensitivity speakers that played much louder than 91 db sensitivity speakers.

How is this possible? Very simple...the 87 db sensitivity had much larger Xmax specs on the drivers....this meant that at high power the drivers were still efficient at turning amplifier power into music whilst the high sensitivity speaker just compressed everything as the driver went outside the linear magnetic field.

So be careful....you can't simply extrapolate speaker sensitivity to dynamic SPL output! (another factor is thremal compression that does not show up in speaker sensitivity)

Audition, audition, audition.....

>3db is barely noticeable<

Not sure I agree with that. 6db is a percieved increase of double. I would say that 3db is noticable.

My recollection is that by definition 1 dB is the threshold of the difference that the human ear can detect, and that 3 dB is double the SPL of 1 dB.

My feeling is that, while it is always preferable for a speaker to have a higher power rating, it does not necessarily equate to the quality of the sound produced. Now, a speaker with more dynamic range will have that quality as one advantage but it could conceiveably lose out on many others. As a personal example I never cared for the sound of Klipsch speakers as much as I did the Quad ESL 57s I owned. But your mileage may vary, especially if dynamic range is at the top of your list.

6db is a percieved increase of double

6db change in spl *IS* a doubling -- whether it's perceived as such or not.
To achieve a 3db increase in SPL you need to double the "power" as warnerwh notes.
In simple terms, +3dB gives you a sqt2 increase in sound pressure level (spl), i.e. ~1,4.

the 87 db sensitivity had much larger Xmax specs on the drivers

Hardly. This has to do with the driver construction -- not its rated sensitivity. And do disabuse yourself: your 87dB driver will need 4x the power to match the 93dB driver. So, on paper alone, at 25W for one means 100W for the other. Don't tempt the voice coils by playing the speaker at 100+ levels for too long or the drivers will kick the bucket (or melt, or both):).
OTOH, we rarely listen at 93+dB continuous -- or do we?