Onyx is def cleaner and technically better overall. I never really liked the VLZ.. it has its uses like Bryan said but I would not want to run everything
through it personally. I think of the VLZ series mostly as a live board first and foremost. For a smaller studio I guess it's not a big deal though. The Onyx board seems more designed as a live board and studio board in both build quality/cosmetics, sound quality, and expandability. Plus you can have the Onyx firewire run tracks from your DAW to the mixer and sum on the board (according to a video I saw).
If you go Onyx you might as well buy the firewire expansion for it as a sound card. Not sure what you are using now though. Personally, I run all of my gear directly into my Mr816CXS interface and don't bother with a mixer. It will just add noise to my signal .. at least at the level I can justify spending money on. Im not packing $ for an SSL
What exactly are you using a mixer for?