Go with the OS 108. They run very sweet. I has a GBM Ultimate with this setup. It ran great, had unlimited vert. I would not pay too much attention about the weight issue. I know have one with a Saito 180 and I would not trade anything for it. As far as weight, well they may come in a little faster, but nothing that an avg. pilot would even care about. The os 108 has lots more power then the 91 and even though it may weigh more, you can plan ahead with servo placement and negate most if not all of any lead placement. My Ultimate has the Saito 180 and the bal. is just fine. When I ran mine with the os 108, I ran a 15x8 apc and it flew great, would climb unlimited and landed very good. These planes have lots of wing area and drag, it's nice to have a little more power. Either engine would fly this bad boy, but I really think you would enjoy the 108. Let me know if you need any more info. BTW, these are great airplanes.
Larry