If you considered last season to be Marbury's worst year of his career, well guess what. If those were Carter's numbers, it would rank in as probably his 5th best season of his career.
Unless there is some Anthony Carter I am unfamiliar with, it is pretty ridiculous to think he would be a better option than Marbury.
If you want a scoring PG, of course Marbs is better at that, but you have to consider what the role requires. Carter is a tough defender, well-schooled in hounding 1s and gamely attempting to bother 2s. He's also a disciplined player, playing mostly to his strengths and leaving the rest of the game to others. He's a proven facilitator, delivering a lot of assists in short periods of time and keeping his turnovers relatively low.
With House, Sheed, Daniels and probably one of the big 3 on the floor, the best back-up PG is one who can get the ball to these gifted scorers in scoring positions. Since he'll be paired with House, this guard should also be able to play tough defense and scrap to make up for the likely size disadvantage.
While Marbury is a gifted player and capable of doing some of these things, Carter's just better at this particular set of skills. Past production shows that Carter has filled this precise role for successful teams, while Marbury struggled in his one chance at it.
That's not to say the Stephon couldn't step up. I would definitely take him. His D wasn't bad, he passed well and his shot should improve, but then again, he's never going to rebound, be a defensive pest or scrap like Carter. It's just not his game.
Stephon would be better off on a team that has a lot of defenders and finishers, but no shotmakers. He would just be adequate on our team.