They are both nice, but I'd choose Vice City. Liberty City looks good (especially with the mod I had installed when I was playing), I liked the downtown with the big stores and the theaters, overall it looked realistic and beautiful at some parts but it may have been a little too depressing for me. Plus you can easilly get lost there and those damn gangs made me give up the game. I couldn't even do some taxi missions- they'd come after me with machine guns and would blow my car up.
Vice City on the other hand was much more fun. It wasn't as big as LC but it was brighter. I didn't get melancholic while driving around. As a matter of fact I loved being a cab driver in VC. Better navigation (I didn't even need the radar from some point on), plus the game had an amazing soundtrack that I have mentally connected to goofing around Vice City. Overall VC beats LC for me because simply, it felt more fun.