The way I see it, there is power and there is weight and there is wing area.
Get the most powerful engine your plane can handle and still have good wing loading. I have the GP YAK54. The wing area is a little low for the weight so I am going to put the OS1.60 in it to minimize wing loading. I have a Brillelli 46 in it now and it flies powerfull but a bit heavy. I do not want even the extra weight of the MOKI. Now the B46 I will put in my brand new GP Ultimat Bipe! The wing area is much greater (300 + sq inches more) than the YAK, and I believe it will handle the engine nicely.

It also depends on what type of flying you plan on doing. If you are doing stalled or near stalled 3D flight you want really light wing loading ( and a really powerful engine). If you are just sport flying or IMAC type stuff then the heavier engine is fine. The YAK tracks very straight lines with my Brillelli in it but it can get ugly in a stall.