We've got threads for both over in OG, for what that's worth. I've got both, but it's been a while since I've played starbound -- it's still sorta' in development, and I've been holding off until it's, well, done. Whenever that happens. Biggest problem with asking over OG is there's been a fair amount of negative opinion build up (mostly for fairly trivial reasons, really) against starbound over the last year or so, and asking there is likely to get folks being jackasses instead of constructive. S'a lot of dross to parse through in regards to more recent stuff on the SB thread, too.
If I had to choose between the two, right here and now, though, I'd probably say Terraria. It's a bit more mature, so to speak, with the primary downside to it being that many of the folks that have it are pretty played out at this point, so multiplayer may be somewhat difficult to get in to. Starbound's a bit more thematically coherent (Terraria's ostensibly fantasy, but functionally all over the goddamn place, while SB's more-or-less sci-fi), but it's also still fairly in flux regarding a fair amount of its design and whatnot (which may or may not be an upside, really). Bonus point to SB, it's a lot more mod friendly than terraria, so if that's an issue for you, you may want to keep that in consideration.
Still, you can't really go wrong with either, imo. Neither's really bad, exactly. One's just in development and the other's, well. Not.