It appears that Mike Carp has wedged himself into a prominent role with the Mariners for the near future. Barring a trade to a new team or an acquisition like Prince Fielder, Carp's performance this year has earned him a right to come into Spring Training next year with a job to lose instead of a chance to win one. A question then might be what job?
As it stands, the most likely case is that Mariners will run with Justin Smoak and Mike Carp as the primary bats at first base and DH next season and the inertia of the situation would pencil Smoak in at first and Carp as the DH, but is that valid? Are there reasons that the reverse would not be more optimal?
In the field, Smoak did come with positive reports on his glove a few years ago. Those reports apparently waffled as he became a Mariner, but to my eyes, and to what most fielding metrics show, Justin Smoak is an adequate first baseman. Maybe not better, but also not worse. For that matter, so is Mike Carp. The samples are too small to be comfortable making a declarative statement, but I can say with confidence that the standards measurements out there do little to distinguish the two in the field. However, I must note that Carp has a Major League sample of about one-fourth as many innings at first as Justin Smoak does.
In general, Carp does offer a little extra flexibility in that we have seen, and Wedge has seen, that he can at least survive in left field for a short stint if called upon. Now, if Carp is starting at DH, then moving him to left field in the event of a needed mid-game shift becomes troublesome because the Mariners would then forfeit the DH. If he were playing 1B, such a move would not be disastrous and it's typically easier to find someone off the bench to fill in at 1B than LF. A counter to that is this is such a rare scenario that it is such a minor probability that it might never occur and it would be almost foolish to even consider it.
Ultimately, because there are so little differences that we can point to in the numbers so far, I believe that the decision would have to come down to subjective measures here. Which player Eric Wedge feels is best equipped to handle playing first base and/or handle DH'ing without losing focus. Another possibility is a mix-and-match DH/1B rotation, but that strikes me as unlikely. I do not expect Carp to get the first base job if simply because, for lack of a better explanation, Justin Smoak got there before he did. Is he the right choice? I don't know.