Manning - Sure he costs a lot, but he also makes guys better. His ability to see the field and get rid of the ball quickly helps the O-line. His accuracy allows a receiver to get a little less separation than normal. Peyton makes plays and he wins games. Still, he's got some of that Marino Mojo. His defenses have not been very good and some defense every year finds an answer for his game.
McNair - He also makes the players around him better, but in a different way than Peyton. If his O-line misses a block, Steve can often make the guy miss or just stand in the pocket, take the hit, and get right up. When McNair rolls out, the DBs have to keep one on Steve or else he can bust a scramble on them. I guess McNair's downside is that he's a good, but not a great pocket passer. But, that's a little overrated in today's NFL (sorry Jaws).
Leftwich - Looks like the real deal after one season. I think he was really helped by having such a strong running game which kept teams from teeing off on him. He's got a good arm and looks very comfortable in the pocket. The vets seemed to respond to him late in the season which shows some leadership. Yeah Byron had some turnover issues, but he should grow out of that. His main downside is lack of mobility, which will only become more prevalent as he gets older and takes more hits (see Drew Bledsoe).
Carr - I shared my gut feelings about David last year on the HPF forum, stemming from a couple of observations from the '02 preseason. I recall Carr throwing a bullet down the middle in the KC game, and it was starkly reminiscent of an Elway, Marino, or Jeff George pass. I realized that ability alone would not make him into a star QB, but it made me take notice. A great or would-be great? The difference is leadership, charisma, guts...It, whatever It is. Some had It (Elway), some never even smelt It (George). The 2nd observation I had about Carr came when he wasn't even on the field. Carr went out in the Buc preseason game with what looked like a bad knee injury. There was a spooky feel in Reliant, like a balloon had burst, and the air had been pulled out of our lungs with it. The offensive line response after that play made me see how the team had acknowledged Carr as the team leader. The line just blew the Tampa D-line off the line of scrimmage as if saying, "If you come after our guy, you will pay". Really, it was the most emotion the O-line showed that season. Carr justified their commitment to him by answering the bell against the cowgirls the following week. And every snap that season. It is in moments like that team chemistry is planted and it has continued to grow (the Falcon game this season, for instance).
Logic says you have to take Manning. The guy is still young and he already looks like one of the great QBs in the game's history. He could get better and that's scary. But my gut says go with Carr. He has the physical gifts. That's undeniable. And it seems to me that he has It. I can't prove with numbers that it will happen, but I truly believe Carr will put it all together and become one of the great QBs (and winners) in this league. My vote will go to David Carr.