Both will make nice, and fairly easy to do conversions. The Ryobi will turn an 18 x 6 prop at around 7500 if you put a little larger carb on it and gut the muffler and provide a bigger outlet. The Poulan will be more powerful, but is a touch heavy. It uses the same basic block as the Poulan 42cc, and weighs about the same. But it is stll worth converting.

AV8TOR

P.S. Here is a pic of my Poulan 42cc when I was putting it on my Giant Stik. I forgot to mention that the Ryobi can be run either with the magneto or with electronic igntion, but the Poulan can only be run converted to electronic ignition because the magneto coil mounts to the saw, not to the engine.