I think Nb4 is a better move as it gains a tempo when the white queen moves. The knight on f8 can move to e6 next move anyway because the black queen is protecting e6.
At the moment the white queen is on a great square, with pressure on the d-file and on two useful diagonals. With Nb4 Black forces White to decide which he wants to keep - will White play Qd2 to keep control of the d-file or Qb3 (for the a2-g8 diagonal)? Neither square is as useful as the one the queen is currently on.
The only thing which makes me a little nervous about Nb4 is that it can be kicked away by c3. Before playing that move Black would need to make sure that the knight wasn't getting trapped and that there aren't any tactics if the knight has to retreat back to c6. e.g. 1. ... Nb4 2. Qd2 Ne6 3. c3 Nc6 4. Qd7+ Kf7 5. Rd6! Black is still winning because he has the get-out-of-jail check Qa1+, but White's position is clearly dangerous and I'd be worried about playing into that position in an OTB game.