I think an important factor is the 1660 is 6gb card while the 5500xt is a 4gb card. So 4gb limits your ability to mine different coins. If you're only talking about mining etc than understandable.
You are wrong here mate, rx5500XT has 8gb versions as well just like how 570 and 580 GPUs have 4gb and 8gb versions too, you can still mine ergo and raven coin using 4gb though but 8gb GPUs are always future proof