The engines in 2750 and 2775 should be exactly the same. Personally, I prefer the the chrome bored Pro295, but BMW and several other premier auto makers use Nikasil in their engines, so I'm sure the difference in durability is negligible for RC duty.

The bearings on the Pro are supposedly a little better also, but it's such a popular size that you can get replacement easily and cheaply.

The carbs are different, though having seen both, I doubt one is better than the other performance-wise. They differ mostly in the throttle and choke lever arrangement and such. Forgot what the 2750 I had came with, but my current Pro295 has the Walbro WT-610.