I went through the same dilemma. I float the top, batting and backing, though, so they are actually only attached to the take up roller.
You will want a flat surface for the actual quilting. In my case, the leader on the take up rail pulls the backing, batting and top under the roller. The floated sandwich then rolls over the top of the 2nd rail, kind of like a waterfall. That gives you a flat (or very near flat) surface for quilting.
Happy quilting!