I've got an Onyx. No experience with VLZ though. Onyx has the 'Perkins EQ' with 2-band sweepable mids which can be nice for a bit extra finetuning or EQ'ed feedback patching, but to be honest I would not buy one for sonics. You're probably better off with A&H in the sound/EQ dept.
The Onyx does give you an immense amount of routing capability (post/pre switching of Aux sends, aux return and sub group routing, etc), but they're prone to develop defects - crackly pots, the dreaded L or R main bus channel conking out (probably due to a master insert jack glitch, some workarounds have been reported), sticky/glitchy buttons (have to wack it pretty hard to unsolo a channel), etc. They're also a nightmare to open up, let alone do any type of internal maintenance or servicing, so I wouldn't even consider it as a fix-er-upper.
I got mine for 250 from a TV studio and for that money it was a no brainer because I needed lots of routing/aux. Anything more, and I would have serious doubts and would be looking for something else.
Did I sell you on the Onyx?
