They are both home replacements, meaning that you *can* have them both installed, but you'll only ever be using one at a time. I have never used Open Home, but I have been using LauncherPro for quite some time.
First off, LauncherPro has two versions - a free one and a plus version that comes with a few extra widgets (meant to replace what you lose when not using the Sense launcher). The plus version is $3, and I think it's well worth it. Open Home is apparently $4 with no free version, so you have only 24 hours to decide if you like it or not.
As far as customization is concerned, I can't say what exactly Open Home can or can't do, as I've never used it. From what I can tell, though, it looks as if most of it's customization is through skins, and various other eye candy. LauncherPro does not support actual one click themes, but you can do a lot of theming on your own to make it look exactly the way you want (e.g. dock backgrounds, different icons, etc.). LauncherPro also has excellent advanced features, especially advanced shortcuts and the ability to resize widgets (with the plus version).
You really have nothing to lose by trying them both out and seeing what you like best. If you like the theming idea, you can also try out ADW launcher. I, however, will always recommend LauncherPro. You'll want to install something like Home Switcher if you intend to test different home replacements. It will make switching back and forth much easier.