RE: How important is barrel length?
Either of the remington's will be fine. Eventually if you like the accuracy game you will need to upgrade to a custom barrel and the Remington is a great platform for a build job. I would say that a 22" barrel would be about perfect for the 308. Get one and shoot the throat out then rebarrel with a Shilen or equivolent.
I'd suggest the varmint model simply because I think the stock is better suited for your purposes.