I've heard also very good things about the Supertigre 45, which speeds up more than the 51.

What I think...is pick up the engine you want the most? do you need nose weight? go for the heavy like super tigre, or maybe SK, or even the magnums.... if you want a lighter take a tower, gms... I don't even know what I'm saying, in this segment, you have plenty of posibilities, pick the one that you want the most.
Engines are pretty running machines anyway...that's why I like model airplanes..because they need engines =)