I bought and played with the 16X18 for about a month before I sold it and bought the 18X20. The 16X18 has more pop and better spin IMHO. I really liked the feel of the 16X18, but occasionally the ball would just fly long (for no good reason...certainly couldn't be my stroke!)

) Usually when you hit the ball, you know if it is going to go way long or not. But the 16X18 would "surprise me" occasionally. I felt it was a bit unpredictable.
I think the dense string pattern centrally compared to the loose string pattern peripherally had something to do with the perceived inconsistency. The 18X20 has a much more uniform string density and pattern and the ball doesn't fly like it did with the 16X18 anywhere near as much. The 18X20 is more predictable than the 16X18 in my opinion. It has more control. But I feel like I'm sacraficing some pop (especially on my serve) and some extra spin.
Overall, the 18X20 suits my game more than the 16X18.
I would string the 18X20 with ALU Power at 58 lbs. on a constant pull machine. I found myself stringing the 16X18 with ALU power but at 63 lbs to try to tame the power a bit.
Hope this helps.