What's better 100w class a or 200w class ab?

An amp with very high rail voltages that is capable of sustained amounts of high current would be best and most versatile. Some loads REQUIRE both aspects for best results. Such a design requires quite a bit of heatsink area though and is not cheap to produce in terms of transformer and amount of output devices. Don't under-estimate the importance of rail voltage in a power amp and simply look for something that is supposedly "high current". You might find something that sounds good, but it would limit what it could be used with in terms of future speaker purchases. Sean
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If you need the power due to inefficient speakers, lossy room and/or high desired listening levels then the higher powered class AB might be better. If you can easily reach desired listening levels in your room with your current equipment and 100 watts of power then you might prefer the lower powered class A amp. The class A amp will probably use more electricity, require more space (may require different cabling) and create more heat.

This is all very interesting, but why don't you just listen to the two ? I would imagine that this question cannot be answered theoretically since it depends on how loud you listen, what type of music, and your preference for the type of sound.

Krell's FPB series amps are designed to run at class A to full power. This might have something to so with why they named them as they did: Full Power Balanced

Richard

I would go for 200w class AB as long as you have at least 8-10w Class A....If the 200w amp has less than that it will likely sound bright after it blasts through the class A region and say a female vocalist is screaming away at the top of her lungs....We at CTC just finished a huge mono amp which runs over 25w class A and, other than running at over 50 degrees C, it measures beautifully when in the class A region with just 2nd and 3rd harmonics and starts picking up some 5th as it crosses over into past 25w up to 400+w or so.....Lots with tradeoffs in amp design and hard to get everything right so we concentrate on the area at 10w and under where most all the music, other than atomic blasts, will be played.....