They are as Larry says, different rifles of
similar design.
When I asked at a show, I was told that the .308 cartridge is too long for the AR mag well, so you will not find any .308 uppers for the AR.
The 7.62 X 39, though, I have seen uppers for that cartridge. I would feel a little weird shooting one of those, though -- as in "which side are you [b]on[/b], man??"
I do not have an AR-10, but shot one once. It wasn't very accurate for some reason, but I think the guy's scope mount (or rings) was loose or something. I didn't notice the brand of scope. Also, he didn't seem to be a guy that shoots a lot.
The .308 is a great cartridge and I'd love to have a bolt action in that cartridge, or even a PRK-correct semi-auto from Browning or Winchester or Remington , though I haven't shopped & don't know what's out there. Of course, as long as I'm in PRK, no AR-10 for me.