I vote to give the Evolution .61 a try. Over the years, it's been reputed to be one of Evolution's best glow engines. My choice was also mitigated by the fact that it's already installed on the plane and ready to fly.

If the Magnum XL .91 RFS and the Evolution .61 were both side by side on your workbench, I'd say go with the Magnum. The Evo is already linked up, so you may as well take it for a spin. The Magnum might give you longer flying times with its four-stroke fuel economy, but overall power and flying characteristics should be similar.