If you want to compare racquets you have to use the same string bed stiffness. When you string the 18x20 on the same tension as the 16x19 your stiffness will be considerably higher and I think you do not want that.
In general you need lower tensions for higher string density.
I assume that you are playing at 37 DT (kg/cm) and when you look at the table below you can see the difference between 16x19 and 18x20.
Measure the length and the width of the stringarea in cm's.