It isn't all about HP, if you have used these motors on similar size hulls you'd know what the differences are.
I've used both many times, even on similar hulls to what the OP has. Thrust has more to do with it than HP, the Yamaha T9.9 has a much larger high thrust prop and a larger gear-case to handle the load, it's designed to push a heavy hull. Merc also makes a high thrust version, so if the Merc is a big foot it might be better, if it's a regular thrust motor a Yamaha T9.9 will be better.
When it comes to actually running the motors, the Yamaha is smoother, quieter, and easier to control the RPM's. The small Mercs have a spot just off idle where the RPM's either go higher or lower, it's very sensitive and very hard to control in that zone, people end up buying electronic throttle controls to make it easier, same goes for the Honda's. The Merc also vibrates, or shakes more at low RPM's than the Yamaha, the shaking can be very annoying at certain RPM's. The Yamaha is also quieter at these RPM's.
Yamaha Kickers out number all other brands combined around here by about 20:1, and there's a good reason for it.
The OP didn't say which models these motors are, so we are just guessing right now.
