so which one should I pick?Assuming they were only used for gaming and were owned since launch, yes, that's where I'd put a used part at. Throwing in a warranty (assuming it's an actual one that isn't littered with "conditions"), that makes it even better.
If you have the budget, always pick the stronger card.so which one should I pick?
Im perfectly happy with these two because of their small amount of power draw compared to other GPU.
I also consider them first - the problem will be if an issue happens and I need to avail their warranty -Yep, not bad prices. I would take the RX 6600 at 180 USD
Also you can take in consideration this cards from Aliexpress, are RX 6600 mobile, with very low consumption power and temperatures, they run without problem with the original AMD drivers and are cheap:
https://s.click.aliexpress.com/e/_Denvwl1