For the purpose you mentioned(trapline) you'd be very hard pressed to beat a 5.5" stainless Single-Six. They're easily found, new, or used, tough as nails, accurate, and very "packable".
The 22 LR cylinder will do fine for dispatching critters in traps, and even if you're targeting bigger(bobcat/coyote) sized critters in your traps, the magnum cylinder loaded up with any of the hotter 22 magnum ammo will take care of that chore easily enough too.
As far as general small game, and plinking, load it up with Wal-Mart special Federal 36g HPs, and you can shoot till your heart's content for about $9 per 550 rounds.
I absolutely love my 5.5" stainless, and wouldn't part with it for the world!