It may not be that one is better than the other except if you are looking at the application(s).
The
HAP-100 is a High Quality Stereo 2 Channel Preamp "and" Class A Headphone amp, so if you want the best of both worlds you employ it.
The
HA-200 is a Headphone Amp "only", is also Class A, but can be bridged with a 2nd
HA-200 for 2 times the power for one of the best Headphone amps available.
Unique to HA-200 is the ability to quadruple the output power by running 'Balanced Headphone Mode', with using 2 HA-200. In the 'Balanced Headphone Mode', simply use the XLR input (one on each HA-200) and output.