in general terms, decreased string density (fewer strings) generates more power while increased string density (more strings) generates more control.
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.
As I am average size, I found it difficult to play all day with a heavy racket. so I use NPS 95, K61 Team and BLX 61 Team, all 289g. Lighter racket also means easier to lift up when playing volley.
here is a review which compares these two rackets.