I’m not an expert on MODx, but I’ve gotten some decent experience with it over the past few weeks. I’m going to say Evolution. Here’s why.
For one thing, when Revolution is out, it will reportedly convert an Evolution site to a Revolution site no problem. Somebody correct me if I’m wrong, but that’s what I’ve heard.
That said, the main problem I ran into when I attempted to get into Revolution at this stage was that it currently lacks a few essential features through addons. Ditto, for one, is not quite ready yet (although there is a getResources snippet that’s worth looking into), and I find that I’m going to need it in just about every project. There definitely aren’t as many addons converted over yet, but there’s still enough to get going, I suppose.
For me, I might use Revolution a little in the beta to get used to it, but I’m going to stick with Evolution for any projects until Revolution has all the features (and more!) of Evolution.
Not to discredit Revolution, though. I love it. I just don’t think it’s quite ready.