I only know the Magnum XL .91 RFS by reputation, and that reputation is very, very good. It's a strong, reliable 4-stroke that is by most reports a very good value. I own a couple of the Magnum XL .52 RFS engines, and I think that they too are strong, reliable, and a great value.

I also own a Thunder Tiger F-91s. It is available from http://www.thundertiger4u.com for $194.99 with free ground shipping included. The Thunder Tiger F-91s is a couple of ounces heavier than the Magnum or O.S. .91 four-strokes, but it is very powerful, reliable, and easy to tune. I really like mine and I won't hesitate to buy another when the right project comes along.

I don't believe that anybody offers a bad choice in four-strokes, it's simply a matter of matching up your project to your budget when deciding which to purchase.

Good luck, and good shopping!