The 630 doesn't have DPLIIx, although you can get an update that's applied with the RS232 port on the back.
The x35 line has one more coax digital input & one more optical digital input. I have a 325 with 2 each, & an extra coax input would be nice, along with an extra s-video input (I don't have an HDTV - yet).
I'm pretty sure the 635 does video conversion. I don't think the 435 does & I know the 630 doesn't.
I use mine alternating as a 7.1 setup & with the sb amps as zone 2. I find the zone 2 to be limited with no video output (I use a chipped Xbox running XBox Media Center), so if you think you might find zone 2 handy, you'll probably want one with video out. None of the mentioned HKs do.
HTH,
Chris