If you want to shoot bullets in the 175gr range, I would get the 1:11.25. Barrel length will be more of a preference. I have seen people easily ring steel at over 1,000 yards with an 18" barrel. I have 308's in 26", 24", and 20". I take the 20" hunting a lot because it is so short and easy to handle. There doesn't seem to be any difference in accuracy between the different length barrels. A lot of this is going to depend on what you really want to do with the rifle. I think a 22" or 24" would be a good all around barrel length.
i was plannin on using it as just a practice rifle and something to shoot ground hogs an cyotes with and to shoot paper i was thinking of semibulding a sps heavy barrel or a howa action but i wasent sure about the twist rate with heavy bullets
My experience has shown that you can sling up to 180grn with a 1:12. If I would have known that would be the absolute weight in bullet that I was going to shoot, I would have gone with a 1:12 in stead of a 1:10. Your pressures will be lower with heavier loads and velocities higher. Here are some of my suggestions:
Any of the Howa's would suffice. The only thing with the Howa's, you will have to get trigger work done for a nice crisp, light pull. The savages would be my first choice. I own a Mod 12 Low Pro. The gun is a dream to shoot.
After experimenting with many 308's and many barrels in various twists and barrel lengths I have settled on 26" for length and 11 or 11.25 for a twist. However I also prefer 168-200 grain pills in the 308 so I use the 11 or 11.25x. If I were going to stop at 180 grain bullets I would choose the 12x. It seems as if you can run top loads more accurately with the 26" than with the shorter barrels whereas shorter barrels prefer loads well under max potential even though physically there is only about 20 FPS gain or loss per inch of barrel max load for max load.