Is there a specific reason you want to go the way of firewire? Computer manufacturers are phasing out firewire for USB and soon to be other connections. Unless there is something specifically limiting you to firewire, I would suggest looking at a USB interface. Companies like RME have made a name for themselves by making USB a very stable and useful connection over firewire.
I've been debating over this hard decision myself with some of the same models. I'm at odds about the following...
Focusrite Saffire PRO 40
PreSonus FireStudio 2626
MOTU 828mkII now the 828mk3 Hybrid
M-Audio ProFire 2626
Overall the PreSonus has all the I/O I want or need but lacks dual headphone outputs. I adds to its favor Studio One (which could come in handy since I'm phasing myself out of Pro Tools), optional remote, and word clock capabilities but the layout of the interface itself is a turnoff as I don't personally like seeing the mic connectors on the front of the unit.
I don't like the metering features/layout on the M-Audio unit (personal opinion) and with AVID starting to leave a bad taste in my mouth I really don't want to have to deal with them with the next phase of my recording system.
The MOTU unit is a tad bit out of my price range and lacks more Mic inputs (I would like to see at least 4) but it has the best rep for compatibility and stability.
Focusrite has the perfect layout and feature set I'm looking for and has some of the best sounding mic preamps on the market (again personal opinion) and also has a nice software bundle to go along with it and it's in my price range
I looked at some other brands as well (Mackie, Apogee, Steinberg, etc) but I kept coming back to these and I think my final decision will be made the day of purchase... I hope you have better luck with your decision than I.