I would've voted Atton. Both are great men in their own right, but Atton has a significant advantage on Carth for me in that most of his flaws are an act while Carth can get genuinely unbearably whiny. Whining and whining and whining about things nobody else reacts the same way to. Plus Atton
captured/killed/tortured Jedi on a regular basis and taught the Exile his techniques.
I'm in the camp that TSL is the better game writing-wise, so there may be a bias there.
Carth's VA > Atton's, though. Atton just sounded horrible and unconvincing when he was revealing his past. Probably should've gotten a more capable actor or rerecorded his lines until he got them right.