"Of course what matters is what happens once the disc has been seated. I just wondered if there was a mechanical effect on the clamping ability of one technology vs. another."
That's why I said the design matters. In my Wadia for example, the loading tray has nothing to do with the performance of the transport. Once the CD is inside the player, its lifted off the tray from the bottom up. The tray isn't used for anything else. It has no effect on sound quality.
My older Wadia 830 had a Pioneer stable platter transport. In that unit, the tray itself was a working part of the transport. (The Pioneer Stable Platter units were the ones with the thick loading tray that you needed to put the CD in upside down.). Once inside, the CD stayed right where it was and the laser read it from the top.
There's several other designs, as well. As far as slot loading transports go, I think your best bet would be to do some research on a model that is in a CD player you are interested in. You need to find out what happens to the disc once it is inside of the machine. That's what really matters.