You can't go wrong with either the OSs. The Saito is a loverly engine, but does take a bit more work to run in, so may not be perfect as a first 4 stroke if you don't have anyone else around to assist (though plenty of threads on RCU covering them).

As for size, as it is a 60 size plane, you would probably be disappointed going for either the 70s in the long run. I would suggest sticking to the 90 size engines so you have enough power to help you progress.

Don't forget the TT 91 also. Haven't had one myself, but not heard any bad comments about them other than they are heavier than the others. (but cheaper).