2) works with any video card with DP so you are not locked own to a specific brand
3) can be moved around to different machines
4) matrox have pretty good support (at least I think so)
5) have been trialed and tested by many users
6) supports SLI/CF
I think generally, once you update the firmware, use certain nV drivers and OS, the system just works, of course with the problems specific to games like Aion etc etc...
But the matrox system just works.
Who knows how buggy the ATi one is going to be, performance of cards etc etc.
I think you got the nail on the head.
For me, i just want 1 screen, 1 card, i wont be interested in more than 1 screen to game on (maybe get a 2nd cheap one for internet whilst playing or something) but thats already possible on existing kit, so sorry if I am biased against more than 1 screen systems :P