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

As an electronics engineer, I ditto Sean. There are few only class A amps out there and virtually none will do 100W in class A. The problem is the BJT saturation currents on the load line. The higher the bias, the smaller the swing allowable and hence the sooner the need to go class B, otherwise your THD goes up several hundred percent. Class B is generally twice as efficient as class A so heats the room half as much.
Capacity on the other hand, has very little to do with class circuit topology. It is dependant on number of BJTs handling current and their ratings. Class B has better current gain than class A but with the right transistors you can do the opposite. Take care. Arthur

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