I like them both...the 12.25 X 3.75 has a stronger pull vertically, and will yank the airplane like a tractor, so it might have the edge there...but the 12 X 4 is a better choice for all around flyability IMO...much quicker throttle response because there is a lot less mass in the outer portion of the blade. (that's why the 12.25 X 3.75 pulls harder, because it has a wider blade )

If you have a heavier airplane, and throttle response is secondary (not flying "3D" type manuvers ) then the 12.25 X 3.75 would probably be your best choice...

If you use a Tower / Aqua Craft muffler you will get a noticable increase in power.
I run mine on 15% w/ an O.S. #8 plug...works well