First off, try a search for RZ there's already tons of stuff on the HQ about it.
In short the RZ engine has the same block as the banshee so it bolts right in. It has a larger head and cooling jackets on the jugs. They also have power valves so the stock HP of the Banshee goes from 37 to 55 for the RZ. That's 50% more HP. The tranny has better shift forks and the ratios are a little different so you can swap 'em out with banshee gears to fine tune your gearing. The third advantage is that if you are looking for a complete engine, they are usually a little cheaper on ebay than banshee engines.
Draw backs... you have to run a battery to operate the power valves as well as the CDI and stator and wiring can be a little difficult you also need a special fitting to convert to Banshee pipes. All in all if you're handy, it's a pretty easy transplant.