On the relative performance chart at https://www.techpowerup.com/gpu-specs/radeon-rx-580.c2938, the GTX 1660 shows 14% higher performance, so it will most likely be better. The 6GB is also smarter in the long run as games are using larger textures than they did 5-7 years ago when 4GB was enough.
Maybe not so on topic but i use a 1080ti system for all my sim racing but just bought a laptop with a 1650 and it plays rF2 and pC2 maxed out in 1080p ! Quite surprised i was . Both cards you mention are faster than the 1650 .