Comparing the game to another isn't possible.
That said, I'm now playing Risen and the combat is very different, that's for sure.
In Divinity 2 you can jump into the middle of a fight and dish out serious amounts of pain, and reasonably expect to survive. In Risen you have to be a lot more careful as even small encounters can kill you if you slack off on your blocks and counters.
I like both games, but I think Divinity takes the cake for pure polish. Everything looks finished and smooth in D2, with absolutely gorgeous 3d models and animation. Risen has somewhat clunky looking models and rigging. On the other hand, Risen seems to have a much bigger world to explore - despite that it takes place on a small island.
Combat in Risen is much closer to realistic fighting than D2, but D2 has a much faster pace to it.
As others have suggested, gameplay videos are the way to go.
