I've heard neither of these amps being talked about here, and I would I think feel a bit better that Ampeg is now part of Yamaha (IF they have extended Yamaha-style customer service to this line).
However, there is ZERO question in my mind that the build quality, performance, and more importantly after-sale service could not be better than Genz Benz and the current Genzler organization: Mr G makes frequent stops here in TB, and his work with
@agedhorse on GenzBenz service (and certainly Andy's work in the Mesa Subway line) would make it very hard for me to consider anything else.
Your results may vary.