I've owned/gigged both those amps for quite awhile now. I had both my 72 SVT and early-90's SVT-II fully serviced when I bought them used, and each is equipped with GE power tubes. My 72 blueline seems to have a bit more of (what I call) a woody tone but that might just be the different preamp tubes. Besides that I cant tell much difference tone or feel wise, they both have about the same level of clean headroom and each sounds great when pushed beyond that.
My amp tech told me quite a few SVT-II models did have some fairly minor issues with plug connections inside. Ampeg even issued an update/instructions back in the day for repairing a common problem with blowing a diode component inside. I'd imagine most all functioning SVT-II's have already had these repairs done by now. You'll find more info about that in my (and my amp tech timv) posts in this thread:
Ampeg SVT-II appreciation thread
I can tell you a 73/74 SVT probably has about $200 to $500 more in value than a SVT-II in similar condition would. And also that my amp tech/buddy commented the SVT-II is much more difficult to take apart and work on than a vintage SVT.
But both of mine are awesome and have worked flawlessly for quite a few years now:
