What do you plan to do with the rifle most of the time? That's a good place to start.
I have both the American and the Varmint. For walking around quite a bit with the rifle- the Varmint is going to get a little heavy. For sticking to one spot, truck bed rail, or a bench- it's fine. For poking in and out of a pickup window on farm and ranch areas- the American makes for a very nice truck gun (that's what I end up taking out the most such as today on a p-dog trip). The American also carries a lot easier and farther without weighing you down, it also has sling swivels. However, a lot of photos I've seen of Varmint models show sling studs, but mine does not even have the holes.
I don't have the carbine, however it does look light and handy. I believe they also have iron sights(?) if that tickles your fancy. In my thought processes, the shorter bbl has a few drawbacks. One is a loss of velocity, however it might not ever be enough for a casual shooter to notice. The other thing is again a personal thing, but I like to be further away from the noisy end of rifles when they go off. You as the shooter might not notice a whole lot of extra noise from a short bbl- but anyone shooting close to you will notice a little. I don't know if I'm articulating those points all that well- but like I said, they're just my thoughts.
Anyway, decide which model/length/weight will serve your purposes the best.