Re: .270 V.S. .308
To try and answer your question, I made the following assumptions:
I used the "best" bullets available with High G7 BC's (by Litz) for paper punching, not necessarily the "best" hunting bullets.
I assumed a minimum of 1000 Foot Lb's of Energy (FPE) necessary for a clean kill on deer.
I used JBM Data and 50 yd increments. I cut off when the data dropped below 1000 FPE.
I used a total of 4 popular LR capable target/hunting rounds rounds: 260 Rem, 270 Win, 7mm-08, and 308 Win.
260 Rem 140 gr A-Max @ 2850 FPS
750 Yds 1059.3 FPE
270 Win 150 Gr Berger Hunting VLD @ 2950 FPS
800 Yds 1004.5 FPE
7mm-08 175 SMK @ 2625 FPS
850 Yds 1041.7 FPE
308 Win 185 Berger 185 Gr LRBT @ 2625
800 Yds 1018.2 FPE
To fully answer your original question, the ballistic difference between the "best" 270 and 308 Win rounds is Nil. Both will get out to 800 yds or so for a "killing shot". And that sir is a pretty fair poke.
The 308 will have considerably longer barrel life than a 270 Win in regards to target shooting. The 270 Win has no dedicated target bullets.
For a dual purpose target/hunting rifle, either the 260 Rem, 7mm-08 or 308 Win have more merit due to better bullet availability, than the 270.
One note: The 270 Win uses a Long action, the others a short action. You should also compare the: 6.5-06, 280 Rem and 30-06. They share the same great bullet selection as the 260 Rem, 7mm-08 and 308, and due to higher initial velocities they will reach out another 50 to 100 yds further, but barrel life as target rounds will be shorter.
Bob