Nash by a mile. First off, Nash is starting while Ray Ray is coming off the bench so I think that makes the choice pretty easy.
If not for that then it's been a long long long time since the lakers have had a legitimate true point, I think regardless if superman comes to the team he makes an immediate impact and immediately elevates everyone's game on the court. If the Lakers can pull of the Dwight trade, it'll almost completely negate any of the defensive holes in this roster because they'll have one of the best help defenders in the league. If they don't, they may have some defensive holes but I still think the improvement you'll see from the entire roster on the offensive side of the ball will be greater than any defensive liabilities along the perimeter.
Ray Allen is basically playing the same role as the players who were there before him, not much change besides a tad more consistency (If Ray Ray can get back to his 100% healthy self again). He's still not gonna see major minutes coming off the bench behind Wade. You might see Wade run point a little bit more this season though.