in general terms, decreased string density (fewer strings) generates more power while increased string density (more strings) generates more control - meaning more time to generate spin.
I have used both 16x18 and 18x20 on a mid-plus and since I am more a serve and volley player, I prefer 18x20 as it gives me very good control when volleying - also less power so where the shot lands can also be very precisely controlled. Meanwhile I also find it easier to spin the ball with a 18x20. But of coz any experienced player can tell how much the ball spins has much more to do with your stroke and follow through than just string pattern e.g. doing a "windscreen wiper" forehand can significantly increase the ball spin when you are closer to the net, making the ball lands much shorter than with a normal forehand stroke.
here is a review which gives your a good comparison on these two string patterns based on the same racket.