I'm not familiar with the M-box but I do have a 1604VLZ. It gives you 16 channel strips including 8 direct outs and 6 Aux Sends including a Control Room and Master output.
By looking at the Mbox site...
Two analog inputs and outputs, featuring Focusrite mic preamps
Pristine 24-bit signal path from input to output
24-bit stereo S/PDIF digital I/O (RCA)
Separate source selection (MIC/LINE/INST) and gain control per channel
48V phantom power
Hi-Z input for instrument pickup
Two analog TRS inserts
When you say 18 channels... those would have to be mixed down in the software, the Mbox only has the 2 in/out selection (not including the S/PDIF which the Mackie has to connection for). You would need the 1604 if you were going to record (live) multiple channels to send to the Mbox, but you would have to mix those multiple channels in the Mackie and send them into one of the 2 inputs on the Mbox thus only giving you 2 tracks on your computer.