RHC, one big advantage for the .308 is the availability of good cheap surplus ammo. I just shot a box of the "British '80s .308" from
http://www.cheaprethandirt.com and it was pretty good stuff. $3.99 a box of 20. And, of course, you can get a wide variety of premium ammo too. My buddy is partial to anything with Nosler Partitions on the business end.
I suppose I must admit that, between .270, .280 or .308 the differences will be rather small. Check the ballistics--they'll look pretty similar. Some folks will swear by one or the other. Hey, that's why they sell so many different kinds! If one caliber was clearly superior, we'd all buy that.
I bought .308 for 3 reasons:
1) Availability of surplus ammo
2) Lots of military/LEO snipers & long-range shooters use it.
3) Short action (shorter bolt throw than .270 or .30-06).
You may have valid reasons for choosing one of the others.
BTW, the Ruger rifles have a fine reputation. No need to drive unless you find a compelling reason to buy another brand. Mine's a Remington 700, but I could not tell you that it is any better than a Ruger.
If your local business is a good business, I'd stick with 'em. They can probably give you some very good advice on scope/ring mountings, which will be as important as the rifle. My local shop put together an unusual scope mount for me that allows me to keep my iron sights, just in case. I'm new enough to this that I could not have figured that out without lots of trial and error.
Please let us know how your new rifle shoots once you get it set up.