I think you summed it up well. Over the past three seasons Carter has total 3.3 fWAR, while Smoak has a dismal +0.1 total fWAR (was actually negative in 2 of 3 of those seasons). But the key to obtaining Carter should be unloading Smoak and not having to eat more than half the $8M owed to him. An earlier post mentioned Carter costing $10M, which I think was assuming Smoak salary of $4M was burned and paying $6M to Carter. They could also get lucky and have someone claim Smoak on waivers.
But I still would prefer Moss, since he has the LH bat and is a decent defensive LFer. If the Jays can get Moss for 1 yr, $8-10M, I would prefer that route