McCourty is already better than Bodden. It's that simple.
McCourty was excellent on the outside last year. He was physical at the line, rerouted WR's to the outside and used the sideline as help, and he came up hard vs the run and tackled well.
McCourty struggled at times when he had to cover the middle of the field. Mike Wallace chewed him a few times over the middle. Against the Vikings, when McCourty moved inside vs Harvin, Harvin chewed him pretty well on a few routes. Even on the strip-interception McCourty had vs Harvin, Harvin was lined up in the slot and beat McCourty pretty cleanly on the route.
It's very rare that an elite corner moves inside and is just as effective as he is outside. Even Rex was hesitant at moving Revis inside and seldom did. The only guy that really does it with a great deal of success is Charles Woodson.
But I don't know why you would want to take McCourty and move him to the slot when we are in nickel or dime. Not only has he shown that he is not as strong when asked to cover in the slot, but he has far and away been the best all around outside CB this team has had since Ty Law. Bodden had a solid year in 2009, but McCourty was a lot better as a rookie than Bodden was in 2010.
Therefore, going into camp, I see it as McCourty as a lock on the outside as our #1 CB. Butler, Wilhite, and Arrington will likely battle for nickel and dime duties. Assuming he's healthy, Bodden has a pretty firm grip on the #2 CB spot and will likely being the season as the starter opposite McCourty.
Dowling is the wildcard here. He really has no experience in the slot, has the build/body type/style of a physical, outside the numbers press corner. You'd think, if he's going to be a regular, he'd be competing with Bodden for snaps at an outside spot.
My guess would be that McCourty and Bodden are the regular starting corners. Butler/Wilhite/Arrington battle it out for slot duties in nickel and dime packages. Dowling is used on on gameplan-based matchup basis.
If we're playing SD and they've got V-Jax, or we're playing the Jets and they have Braylon/Moss/Plax, Dowling could figure into the gameplan in a big way with his ability to cover taller/bigger/more physical receivers. If we're playing the Jets and are having difficulty covering Keller or we're playing the Colts and don't have any LB or S that can really hang with Dallas Clark, Dowling could be used to match up with those "move TE's" who are difficult to cover. He brings a tremendous amount of versatility.