I second the Saito .91 That's what's on mine and I LOVE it. However, I have friends with same plane and a Saito 100, and another with a YS 91 and they both have near unlimited vertical.
Mine will hold a hover, but struggles when pulling out, where theirs are like mine but on steroids. Give er a little throttle and it's bye bye It all depends on your flying style, the smaller engines will fly the plane on the wing great, but the more powerful can fly it on the prop when needed.

If you are the "Tim Allen" type (arrr arrr arrr ) go for the extra power, you wont be sorry.

Oh, and for what it's worth, my 4* is easier to fly than my trainer was. I like it so much, I am building another one this winter so it will be ready when the "expiration date " comes around on this one.
Dwight Hayden