I'm not enormously keen on the idea of either playing too regularly, both are merely fingerspinners.
TBH, I think against poor players of slow bowling (not neccessarily spin), which most West Indians certainly were last time they faced England (TBF, of course, they were poor against both slow and quick which was anything above woeful standard - ie, not the sort of stuff Harmison and Plunkett served-up), MSP is likely to do far better than Swann. This is turning pitch or no turning pitch, but more so than ever on a turner.
Also, MSP at his best > Swann at his best, on turning pitches, against any batsmen, for mine. Can't ever see Swann producing a spell like MSP's second-innings one against NZ in 2008. However, Swann has already slightly exceeded my expectations so far on this tour and this game isn't over yet so he has the chance to do so more.
I wonder if Swann is a bit better than I, or a few others, have realised. Having obviously never seen him bowl in the longer game, and given that turning surfaces at Trent Bridge (unlike Wantage Road, his old home ground) are pretty unusual, no-one who isn't a religious Northants or Notts fan who goes to near enough every game will be able to know for sure exactly how good a bowler he is.
I think, though, that he's already shown everyone that they can be enormously relieved the selectors had enough faith in him to pick him over Rashid for this tour.