Canon doesn't really do 'stealth' updates - doing so would mean changing the manufacturing line, requiring revalidation of the process, etc. If there are issues, yes they change the line - but there are no advisories on the 300/2.8L IS. So, the electronics will almost certainly be the same between the two lenses.
I'd be inclined to the pristine US over the 'very good' UV, personally. I have had a UT 300/4L IS and an UL 200/2.8L II, and both were excellent.