only drawback to the 91/30 vs a carbine like an m44 I can think of is the extra weight of the full size rifle. The Mosins are relatively heavy guns to begin with, so the longer ones are heavier. Not a big deal really, unless you use one to do hunting where you have to carry it all day though, and even then, just using the sling probly eliminates that mostly.
Oh, another drawback is the full size are a little quiter, and dont have the giant fireball coming out the barrel, so if that kind of thing makes you smile, then it may be a drawback.
Otherwise, the full size seem to generally be a little more accurate I'm told (only have 2 carbines, M44 and Chineses type 53, so I cant say from personal experiance, just seems to be waht I hear), and the added weight may help with recoil some, (but a slip-on buttpad makes the point moot), and added barrel length probly adds at least a little extar MV (not sure if it's enough to really matter or not though.).
Generally, the rifle vs carbine choice in Mosins seems to be based more on personal preferance than a plus/minus logic decision for most people.
either one you choose, they are basically a whittled down railroad tie with a barrel and bayonet
that you'll be REALLY hard pressed to damage the value, function, or looks on (with rare exceptions for the rare(er) mosin types)