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

Most amps that are rated for Class A operation are actually AB. They stay in Class A much longer than normal, so they refer to them as Class A rather than stating "VERY rich Class AB". Very "rich" AB amps stay in Class A for anywhere between 8 - 30 wpc ( some go a little higher ) before switching over to Class B operation for power levels beyond that point. To dissipate BIG wattage and run Class A would take MEGA heatsinking and forced air cooling if you were standing on the throttle for any amount of time. You will realize just how much heat you have pouring off of one of these amps as the ambient air temperature of the room will climb quite noticeably.

As to whether it is important that the amp stay in Class A for a long time, some people say that Class B sounds better. While there are technical reasons as to why this would be impossible, my ears tell me that the higher the amp is biased, the better it sounds. Keep in mind that high bias amps typically require very long warm up periods to sound their best and typically perform optimally after being on for quite some time. As such, i don't shut my amps off as i've found them to really "sing" after being on for about 2 - 3 days or so.

As to your original question, unless the circuits were identical and you knew the point that the amp switched from Class A to B, there really is no "best" answer. Sean
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Try to find out what's the peak and continous current rating of each. Select the one with highest current capability. Most of the time the high current pure Class A amp will outperforn the low current high wattage Class A/B amp.

I would have to agree with Audiomax....current drives the loudspeaker. Amperage, not wattage is the key. I have heard some rather gawdawful hi-wattage amps, and have also experienced the fidelity and depth of sound that that a high-current, though lower wattage device can provide.

If you can, try to listen to both amps under identical conditions...same program material, same preamp, same cables, same speakers. That is, if you can... this is not a perfect world.

Try to find out the output at clipping. There are a lot of amps out there that will drive demanding speakers at > 4 ohms continuous output. The question is: how will it handle the short term dips below 1 ohm? Regardless of impedence ratings (which are at continuous output), all speakers will provide short term loads of under 1 ohm (drum beats, bass plucks, etc).

I just upgraded from a Plinius 8200 (175 Watts into 8 ohm) to a Plinius SA-102 (125 Watts into 8 ohm). While the 8200 is a great amp for the price, the SA-102 is heaven. According to the Plinius US Rep, the 8200 pushes about 1000 watts at clipping and the SA-102 pushes about 4500 watts at clipping! (my numbers might be off a bit, but they are in the ball park) When I installed the SA-102, the difference in slam, speed, bass and overall speaker control was dramatic.

What I don't understand is whether the increased capacity of the class A SA-102 is due primarily to the class A electronics, or are class and capacity independent specs?

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