My late wife and I were KenQuilt dealers and for the money, I don't think that they can be beat. They have added a computerized system, but I don't care for it very much. Gamble has the best computer system, in my opinion, if you can afford it.
Most of the systems have a 18", a 23" and a longer one. I'd say your best one is the 23", because with thee shorter ones you can't cover as much material in one sweep, and the longest one dosn't do much good if you have to extend your arms to far.
Oh, I almost forgot. The height of the neck is just as important as the length. You have to be able to roll the quilt up and not touch the neck. Many of the new 18" machines don't have the space for a king size quilt.
Therefore, I reccommend a 23" with a 12 foot table.
Good luck,
Lanny