I've been in this situation a few times and ALWAYS regretted selling any gun. I'm currently selling an 870 rifled barrel, like you have, since it just wasn't being used. I love my current deer rifle, which your .308 would probably be, and just don't have a need for the 870 rifled barrel anymore. However, I wouldn't get rid of the entire shotgun. Believe you me, you will wish you had it 1, 2, or 5 years down the road.
Sell off the barrel and any accessories you just won't use and use that extra $$ to justify the .308 purchase....and the extras you'll put into that rifle.