In almost all cases, bigger flies better. I built a 4*60 for my son last year and in my 25+ years of modelling, it was one of the nicest flying planes I've ever flown. The .60 size will be a little more stable than the smaller one. However, I'm sure you'd be happy with either. If you're going to get a new engine, you may as well get the larger 4*. A good .91 4 stroke is perfect for it.