I don't know which one to get for my first build
590, Runs hot, Runs loud, runs lower performance on average, you get the free game+game pass for 3 months.
1660, No free game, Little more performance, Little more expensive,
runs cooler and quieter.
Both have points for and against them that can make or break them.
Having worked with both, the 590 isn't as terrible as some but without the extra's isn't worth saving the $10 on, with the extra's it's debatable, the 1660 is hands down the better card across the board and worth the extra $10, but your not going to get the extra's.
To add to Gilroar's statement, FreeSync monitors are also cheaper, and can work off HDMI.
(NVIDIA cards can only utilise DisplayPort for frame sync, so you'll have to use V-Sync or FastSync)
If you go down the AMD route, you can get FreeSync panels (with HDMI) fairly inexpensively (this Acer screen is ariund $90)
Freesync hasn't worked 100% over HDMI with Polaris on many panels for the last several driver updates since Navi launched.
It's been an ongoing issue that AMD hasn't managed a fix yet other then suggesting reverting to older drivers.
Defiantly worth the extra few bucks for a panel with a Display Port so you don't limit yourself later for a feature that driver updates can make or break.
Like I did.
Both great for 1080. The 580/590 have been discounted likely due to new 5500 coming maybe around end of year nobody is sure, but they do take more power and older design. May not be the best if you value a quiet system. They might go lower in price yet hard to say.
the GTX 1660 is overall better
It is little better performance and less power consumption but 580/590 both do well at 1080. Right now on newegg its RX 590 $195 vs GTX 1660 $230 cheapest ones, $180 for a RX 580 8gb. Its kind of a wash. GTX 1060 6gb for 209.
$210 on Amazon.
Good find, it says available in 1-2 days. Its the single fan model but they say temps are not a problem with the 1660 anyway.