For the money, it is hard to beat the ST G-51. I've got about four gallons through mine running it with a Macs muffled tune pipe and header. These engines like the RPMs and to me the best props for the G-51 are:
[ul][*] Master Airscrew Scimitar S2 11x6 - flexible blades allow it to spin up a bit more.[*] APC 11x5 - 11x6 just won't let the engine get into the sweet spot.[*] APC 10x6 to 10x8 - 10x8 on a pipe, may be too much for the stock exhaust.[*] APC 12.25x3.75 - for 3D and good climbing
[/ul]

You need to prop this engine to run at least 14,000 on the ground to get into the sweet spot - IMO.

Currently I run an APC 10x8 at 14,200-14,400 with a pipe and 15% fuel. It will turn a 10x7 a bit over 15,000 but the vibration and noise was a bit much for me.

I also have a Jett muffler that I have run on it with equally good results. Not as good as a well tuned pipe but close and a lot less hassle. The Jett is a bit louder than a Macs muffled pipe though.

I have a love/hate relationship with the ST carburetors at times, but right now mine seems to be running about perfect. It takes some getting used to. OS carbs are almost set and forget (except for the occasional high speed adjustement for weather conditions).

I would love to try a G-90 some day if I ever buy a plane that needs that sized engine.