Thanks for the answers. I agree with alacoume, the planets are probably more interesting but for collection purposes, I can't really afford the whole set. After doing more research, I think I'm just going to get the crazy 4x4 2- I could just use it as a 3x3 first.
But once you have, say, mercury and jupiter, you automatically have the whole set if you want. Because you then have 6 of each kind of center. So you can then make all the others, saturn, mars, the whole lot (plus you get your circle cube). It's a pretty fantastic investment for buying 2 cubes.