Sounds like you found some good deals regardless.
I mean, 2K20 has more mods right now, but as you can see plenty is being made for 2K21. Some people are bringing 2K20 up to date by making cyberfaces of rookies from scratch, and doing conversions from 2K21 as well. However, you may not have a full release as quick as you may like, as conversions and faces, as well as other aspects do take work.
Gameplay wise, I think 2K20 might be a little more well rounded. 2K21 has a slight advantage with dribbling mainly because it uses the traditional size up system. I have gameplay videos of both 2K20 and 2K21 on my youtube, here are a few of them.
If you are interested in retro, more retro mods and rosters exist for 2K20. If you are looking to get up to date FAST though (As in, you are interested in the most up to date rosters because your priority is the now NBA), it may be worth giving 2K21 a look.
I know Andrew enjoyed NBA 2K20 for the most part.