Though they are both pretty good engines, I would definitely go with the Ryobi personally. Is this for airplane or weedeater use? Is the Ryobi rear exhaust/rear intake, or side exhaust/side intake?

In the smaller sizes, the "Weedeater" made by Poulan is a very cheaply made engine. In the 30'ish size and up I think they are better made.

AV8TOR