Agreed, an inverted mount is best cosmetically for this airplane. Both of mine were/are side-mounts, because I'm basically lazy... and the Laser's carb would be too low to work without a pump or regulator.

IMO, an OS 1.20 four stroke is just the right amount of power for the CG Ultimate (my Laser 150 approximates this power level). YS are a lot stronger; I had an old 120N in a GP Super Aeromaster that was absolutely insane on 30% heli fuel.

Given your choices, I would grab the non-air chamber version (its probably a 120N). Its the only YS I have owned that didn't eventually give me problems. While it may not be quite as strong as the AC, it is a solid brute of an engine.