alextlit,
They are pretty similar except for the string pattern. They have the same swing weight (which is too high for a lot of people), weigh the same and feel pretty similar in play. I think the 16x18 definitely gives a bit more spin but is not as consistent in its response as compared to the 18x20 version. I prefer and play with the 18x20 version because it has a more consistent feel from the string bed, offers a bit more control (to me) and doesn’t seem to have any shots that “fly” on you like the 16x18 version does on occasion. The extra strings may help your serves out wide if you have the racquet head speed as well. Also, you can generate all the spin you need with the 18x20 version so don’t believe the hype that your spin will reduce dramatically, if you have good racquet head speed.
Yes, I could play with either but just prefer the tighter string bed for the added control I feel it offers. The difference is pretty subtle so unless you are an advanced player you may not notice much difference. If you can handle the swing weight, they are great sticks. Touch is definitely more sensitive and harder in the beginning because of the extra stiffness, especially if you are coming from a more flexible stick (like I did).
I don’t see many of them around myself and I am the only one who uses the 18x20 version in my area that I know of because I had to special order it and know most of the better players around. A long play test is a must to make sure you don’t develop arm problems after extended play.
Good luck with your decision. You really can't go wrong with either.
TM