You may have made your decision already but here goes.
I have the original mackie onyx. The two should be pretty much the same. The one difference is the firewire capabilities.
As far as sound goes, there shouldn't be too big a difference between
the studiolive and the onyx. Though, personally, I preferred the onyx. It is a fully analog board with a separate digital component.
The studiolive is fully digital.
So, if you're looking for a board with everything built in. FX and compression.
The studiolives fat channel might be for you.
Personally, I like having knobs for outboard things like EQ and compression as opposed to a digital readout. Also, I had heard bad things about the presonus converters on another board.
I've had good results with the onyx but I barely use it now for recording because I have built up a bunch of outboard pre's. I still, however, use it live and its a tank. I've never done sound for a gig where the onyx was not enough. Its usually too much.
