Go for the full version (7). So long as the person you're buying from hasn't already used that serial no to upgrade (a serial number can only be upgraded once).
If it has been upgraded the choice is a bit trickier as when/if you ever want Vegas 8/9/10 whatever, you'll have to buy from scratch. If you bought MS8 now you'd probably (if Sony follow the same paths as previously) get an upgrade price.
I'd probably still go for 7 if the cost is low enough (ie less than MS

I can't think of anything in MS8 that would be an advantage over Full 7, apart from maybe the tutorial which are only any use once or twice while you're learning.