It's not VCR vs. TBC, but VCR (with internal TBC) with external TBC. You need both.
- The AVT-8710 is an external TBC.
- The AG-1980P is a VCR with internal TBC.
The different TBCs do different things.
I know, it's confusing, but the term "TBC" is too loose. So loose, in fact, that some things claiming to be "TBC" are really not (example: Canopus AVVC-300, which has a "TBC" that does nothing at all, so therefore is NOT really a TBC at all).
A quality setup is about $1k. You can always buy it, use it, and resell it. Don't buy on eBay (you'll usually be sorry, ESPECIALLY with Panasonic S-VHS decks!).
Don't forget to look in the marketplace forum:
http://www.digitalFAQ.com/forum/marketplace/
I have one AG-1980P for $375, and some DataVideo TBCs for about $400 (not yet listed).
If you want some good gear, then get those.
The AVT-8710 hasn't been good since 2010. Don't buy the black boxes. Only the green ones are any good.
I'm entirely against buying a cheap doodad and using an old VCR. The quality will be utter crap, so why bother transferring it at all if it doesn't matter?
For only 20 tapes, we can probably do it for less than what you'd spend in DIY hardware, and with zero of your time required.