Ah, Balmorra. A once-beautiful land marred by bloody warfare. I hated it. For me, Balmorra ranks about as far down as Hutta on the list of planets I enjoy visiting. The place is a barren wasteland. Expect a LOT of walking. On top of that, one of your main Imperial contacts there is extremely irritating and full of himself (moreso even than most Sith lords!).
Nar Shaddaa, in contrast, was one of my favorite planets ever! The plotlines were captivating and I felt the scenery and feel of the place were perfectly spot on. Standing in the Promenade was like standing in downtown New York or Beijing. Huge signs and ads, glowing lights, savvy vendors, and people everywhere. Plus there is a sidestory relating to Revan (not like those crazy Revanites) which KotOR veterans will appreciate.
For those of you on PVP servers, this is about when you will need to start watching out for the opposite faction. Some zones are shared by both sides so keep your eyes peeled for those Republic scum. Sorry for the short post but there isn't too much to say about these places without spoilers.