I prefer Graham Norton.
Alan Carr is cool as well, but I get the impression that it is for a younger audience. If you just check out the guest list, there is an influx of younger guests (Cast of Twilight, X Factor participants....) and the official Twitter page for the show is all teenagers wanting to know if they can get tickets or where the best place to stand outside the studio for autographs.
Graham does have younger guests from time to time, but he does not have them on constantly. His show does seem to be for a more mature audience.
Also, I kind of prefer the dinner party feel of Norton's show, having all three guests together, instead of bringing them on one by one., (in seperate interviews) You can't also have banRAB or big casts on Norton's show. On Alan Carr's show you would get three actors from Glee, but on Norton you would only have 1.
But that's just my opinion!