For driven I have a 9.3x62, & a 9.3x74R double rifle. I started with 286g Oryx in the 9.3x74R, because that's what I had it regulated to, but having shot some Geco 255g, I bought for practice, it shot as well, if not better than the Oryx, was a fraction of the price, and dropped the boar it their tracks.
So when I got the 9.3x62, I tried the Geco 255g, and it worked well in that too. That said, Like Orion, I've also started to load with the 232g Oryx, which is more than capable of putting any boar down, or big Red stag down
but shoots a little faster, and feels a little softer.
I would not use either of these shooting from a seat, because whilst shooting driven, and shooting stood up, I'm oblivious of the recoil, even shooting a string of six shots, but the same isn't true shooting from the bench, and I think they could be pretty uncomfortable from a seat, where you may have to shoot from all manner of angles.
I currently use a 7x64 from a seat, and I would have thought your 6.5x55 would be perfect from a seat too.
Personally, I would pick a 30-06 over a 308, if you don't like the idea of a 9.3. Also, I would suggest getting it conditioned for use in the UK, because if you say it's only for use abroad, that's what they'll give you, and you never know if you'll need it over here !