several advantages: Ammo is cheaper, easier on recoil, a good carbine is very affordable. Good for cqb, not so good for distance shots though.
normally using a pistol caliber carbine is frowned upon for many reasons for a shtf type of situation.
1) they are approx the size of some rifle caliber carbines. and if you are gonna have something that size you might as well have a rifle.
2) a rifle is generally better to have if applicable in a situation, not only being a rifle but the rifle calibers.
3)there are gonna be better results with a rifle caliber carbine than with a pistol caliber.
4)yes the barrel is longer so there will be incresed velocity and ft lbs of energy delivered on target ie better terminal ballistics.
5)they are easier to shoot and making accurate shots compared to a handgun come much easier, but again, why hit them with a pistol caliber when you could have a .223/5.56 etc.
Pro's:
like you said ammo is cheaper and that is definetly a plus these days. they are great for plinking, just to have and such, and they are great for high volumes of trainning. learn the basics and do the drills and go through the high rd counts with the less expensive ammo and weapon, and then implimint what you learned with that to the other rifles that you would actually want to use in a shtf sittuation.
then there are people that believe that you should train how you fight. and suggest that you train with the weapon you are gonna use and not doing what i stated above, i am not in that group.
i am not trying to tell you not to get one because there are a few that i love and would love to own, but i wouldn't use it as my shtf gun especially as the primary and not even the secondary that is what my ar and 870 and ak are for, they are alot of fun though.
recoil, is not much of an issue with an ar either, some ak's are better in this department than others as well, the yugo underfolders are very soft shooters.
accuracy, you should be able to tag and get good hits on targets at 100yds and at 50yds all day long pretty rapidly, but again with a rifle you will be able to shoot out farther, and you will have better terminal ballistics once the rifle round hits the target than if you were using a pistol caliber.
btw any thing that you train with and are profecient with is better than everything else. no matter what it is. if you have little or no experience with it and no trainning.
my choice in shtf
1)ar/ak
2)pistol caliber carbine
3) handgun