For me 'overpowering' a plane is fun to 3D or just to get super nice uplines. I got a ST90 and to be frank I don't think much of it. For me a 60 2C and a 90 2C usually is using the same block and the weight is about the same. Hence, I would go for the 90 2C and not a 75 2C. I ran my WM Super Chipmunk with the ST90 and it was not very impressive. If money is not an issue then go with something like the Saito 100, light with a good punch. That would be great on something like 4* 60 ... you can fly her doing impressive big loops.

The Saito 100 is a very versatile engine, you can use that for quite a few planes from aerobatics to 3D. The ASP or TT 90 sized engines are really good stuff, less hassles and they run very well. Think about it ...