Yes, all three of the Mackie Onyx mixers can take an optional firewire interface..... but IMHO, the firewire is much better for live sound with a little recording on the side, than for use in a recording studio.
The reason I say that is that the firewire interface takes the signal RIGHT after the preamp -- before the EQ, even before the channel inserts. In other words, if you use the firewire interface, you can't patch any external equipment, such as a compressor, into the signal chain. Yes, its a nice clean signal to the computer, but it doesn't have any good routing flexibility. The direct outs take the signal from the same point, but at least you can get to the signal on its way to the computer.
On the other hand, the sound is great, so I'm still liking it. I run the direct outs into the MOTU. Typically, I don't use any outboard gear in the chain - I prefer to record dry and add effects later. Given a good 24 bit dynamic range, it works pretty well.
-lee-