I would go with the OS 91 fx, no reason to go with the 61. The two motors weight the same, although the muffler for the 91 weighs a bit more. The only time I would go with the 61 is if the plane couldn't handle the power of a 91, the U-Can-Do is no problem with the 91.

I use a 15x4 APC prop on my U-Can-Do 60 with OS 91fx. It turns around 10,500 peak, which I back off to around 10,300. Lots of power - when my 91 is running right it pulls out very strongly from a hover. Lots of people report problems with the 91, I didn't have any problems for the first 100+ flights, but I am now starting to have problems, although it may just need some tuning (the muffler was coming loose, which I didn't realise, so I messed around with the low & hight speed settings).

I think you will love this plane. It is my first 3D plane. Very good for hovering. Other manuvers I have been trying include inverted flat spins (very good), knife edge spins (these look really impressive to me, especially with a fair bit of power added).

Malcolm