I'd say the 1660 just because it should perform slightly better, is a newer model, and will cost only $20-$30 more in general. I think the 1660 are already out too, as newegg has listings for some right now.
You should also keep an eye out for some second hand cards on ebay or similar places. For example ebay occasionally has offers where you can get 10%-20% off of anything on their site, I used that to get a second-hand 1070 for $250 a year ago. Just make sure to look over the seller's reviews.
How long until the 1660 comes out?
stop supporting nvidia
What's so bad about them? I see hate for them all the time, but can never figure out for what reason
check if you have a good enough PSU