From my use of both of them I prefer the AutoRifle. Its accurate, has a large magazine and ammo capacity(100/400 as opposed to 30/120) and it can actually do soft damage to several vehicles(Which can come useful in certain cases).
I almost never buy the carbine, it's recoil is too horizontal and as such feels incredibly unpredictable. Maybe I'm missing some nuance, but that's a deal breaker for me.
It's an OK supplemental weapon but I think the AutoRifle is competitive enough in comparison.
As a sidenote, the whole "It's OK if it's shit because it's free" argument is valid to a certain extent. Economy is a part of the game, you use your credits to buy better stuff. Why spend the credits if it doesn't get you anything? But that doesn't mean it should be imbalanced - the Carbine ought to be better at what it's designed to do, kill infantry. Nothing else.
As a Soldier(Which I play pretty consistently except to switch to Engi to repair buildings/kill beacons) I pretty much never buy it, in any case.