A 20" barrel is fine for a 270, but a good argument could be made that the 7-08 is a better choice in a carbine.
Any noticeable difference in performance between longer vs shorter barrels or between the two on target will be undetectable, but when comparing the two across just about all other performance factors, balance, lighter weight, less muzzle blast, less recoil...etc., the 7-08 has the edge.
Also, at some point, the better bullets available in 7mm will exceed the performance of the 270 downrange, so your choice depends on what you want to do with the rifle.
If you're not a reloader, ammo selection may be a factor for you, but in the end, it will make little or no difference on field results.
