Much of it depends on what type of aircraft you plan to install the engine in. The RIRE Ryobi lends itself very well to planes with long narrow noses like the PT-19 and J-3 Cubs. But the drawback is that the firewall-to-prop-hub distance is very long with most of the commercially available mounts for the Ryobi ( typically >7-1/4"). This means you'll either have to move the firewall back a couple of inches (easy to do with a kit, not quite so easy with an ARF) or mount the engine with the carb sticking back through the firewall.

Not sure which Mac you have, but I think it's either SISE or SIRE, so the firewall-to-prophub distance will be more reasonable, but it will need a wider cowl to conceal everything.

Powerwise, if your Ryobis are the newer dual ring versions, then they will probably be comparable to the Mac. Otherwise, the Mac will be a substantially more powerful, albeit heavier, engine.

If this is your first conversion, and you plan to do most of the work yourself, I would recommend the Ryobi. It is a more popular choice for conversion, so both sources for parts and the experience knowledge base will be more plentiful.