Rough Rider Pro, as noted above, is the fancier, paid version. It's three Rough Rider compressors in one package, which can either be configured as a multi-band comp or as three broadband comps in series.
It also adds an important feature that the free version lacks: labels on the controls. With it, you can put in specific ratios and attack and release times, whereas the freebie makes you guess.
You may have noticed that both free and paid versions offer compression ratios up to 1000 (!) to 1. I can't imagine what one would need that for, but somebody must have figured out a reason for it. Serious audio damage would be my guess.
Both versions offer a minimum attack time of 100 us, so this caveat applies to both: if you turn the attack all the way counter-clockwise it's gonna distort! I am running it with a fast attack (not minimum but actual time is unknown due to lack of labels), which is undoubtedly why I'm hearing distortion. However, as I said before, the distortion isn't unpleasant. Crunchy, like you'd expect from a field-effect transistor. Or a Green Day lead vocal.