Stick with the older in both cases. The newer Ryobis have some slots in the cylinder that cost some power. We think they are for smog purposes. The newest Ryobis also have a different crankcase design that is harder to convert. Also, an older engine is more likely to have a carb with both high and low adjustments. Many of the newer engines, thanks to the "all knowing" EPA aren't fully user adjustable.

AV8TOR