On the few I've compared, hull and liner on the 20 feels more formidable, but not without problems. The V-21 is more clever in the details --baitwells, transom, etc.
There is no noticeable freeboard change between the two models per se. The deck on the V-21 is raised to avoid the foot-soaking problem that comes with saturated hulls, heavy-weight engines that older V-20s face. Hunt and others (witness the Moesly-inspired Seacraft 20 issues on classicseacraft.com) were known to place the deck perilously close to the waterline. Put four fat guys on the stern and you'll *have cold feet as water rises up through the scuppers.
V-21 doesn't have this problem, but as a result of the raised deck in the V-21, you have lower gunwales, so smaller kids have a lower "safety-rail" between them and the water, but this is a benefit to anglers gaffing a fish. Ahh, everything is a trade off.
BTW I did come across a pristine 87 V-20 with 2003 power. Gentleman is clearly anal about it. Poked around in the bilge and the foam around the tank was dry as if it had come out of the factory. Not a waterstain on the exposed wood. Stainless and gelcoat sparkled, but he's asking way too much. I'll divulge the locale once I'm convinced *he won't come down into my range.
Still looking. Aiming for a decent boat with no major problems except the engine, since I'll likely repower it.

Cheers