I'd opt for the 12' table. Remember that even though the rails on a ten footer are 120 inches long, you won't be able to quilt a quilt with a back larger than about 106 to 108 inches. You are never able to quilt the full length of the rails, because of the width of the machine. So the biggest quilt top that you can do will be about 100 inches wide, since the top has to be a few inches smaller than the backing to be longarm quilted.
My humble opinion is to get the longest table your quilting room can accomodate.