I don't see that an additional 1.5" of barrel (going from the carbine to the para barrel) would make the rifle any more effective. The velocity gain might well be within the standard deviation of a lot of surplus ammo. If you handload (which given .308 ammo prices isn't a bad idea anyway) you can tune the ammo to make the most out of your barrel. A lot of hunters have taken a lot of game with .308 T/C pistols with 14" barrels.
If you want an intermediate rifle, get it in an intermediate cartridge, like the 5.56x45NATO or 7.62x39R
Nah. I've shot steel targets with both types of rifles. .223 makes a "ping". .308 makes a "thwack". If someone was trying to kill me, I'd rather thwack them than ping them, regardless of barrel length.
I think the idea that a .308 rifle
needs a long, unweildy barrel to be effective is something of a myth*. As I said, the SCAR-H, going to SOCOM, in its standard "battle rifle" variant has a barrel all of sixteen inches long. Only the "sharpshooter" barrel is longer, and that's only 20". The M110 sniper rifle the Army just adopted sports only a 20" tube, and this weapon is replacing large numbers of M24 SWS rifles in service.
*I have heard, however, that properly stabilizing 175 grain match ammunition for long-ranged shooting requires 18" of barrel. I don't know if this is true or not. For me, personally, it's not an issue; a FAL with an Aimpoint isn't exactly the type of weapon you run a lot of 175grn Gold Medal through.
.223 is a LOT more dependent on velocity for effectiveness than .308, and yet people have no trouble chopping it down to sixteen, fourteen, even twelve inches.
Some rifles look/handle better with an 18" barrel, though. The Springfield SOCOM looks funny to me, but the Scout-Squad looks nice. It might come down to which barrel length and weapon you feel fits you best. That's the most important consideration.