RE: Filling in a 4*60
So far the magnum 4 strokes I have had (.61 and .91) are working great and I've got them both on sale for killer prices.
That being said I think either engine you mentioned will be good - the .75 might be a little better and I've read good things about it in the engine forum.
I just won a 4*40 ARF at the club meeting raffle last night....