The RPs are a paint that does not dry on non-porous surfaces. Since it's already in liquid forms, it's pretty easy to mix colors. I like to use a CD as a palette, and when I'm done painting I just pick up all the "leftovers" with a brayer and go over a few pieces of cardstock to use as future backgrounds. They don't really dry, but are absorbed, which is why they really need to be used on matte cardstock.
The TH2Os are shimmery cake that you add water to - I generally use them with a waterbrush. They're very pretty, but in general I find that they provide less coverage than the RPs. Not that that's a bad thing; just a different, sheerer look.
I have and use both, depending on the project. I like the deeper color I get with the RPs, but I hate waiting for them to dry (which can take awhile!) I love the portability of the H2Os, and the fact that there's no chance of me dumping paint all over my floor.
Here are some good tips for the H2Os:
http://www.rubberstampsclub.com/tips...ling-h20s.html
and Radiant Pearls:
http://www.rubberstampsclub.com/tips...nt-pearls