Check out this thread:
I have the Orion U-mount. I debated between that and the APM mount and went with the Orion because I found it on sale for $429 (regular price is $500), vs. $650 for the APM mount.
I like the Orion U-mount, except it is a bit too stiff (too much friction) on one direction, as discussed in this thread. It is no a big problem for me, I still like the mount, but I wish it had less friction. It appears that the APM mount is better in that respect.