The Presonus has the best build quality and a hell of alot of inputs (6+2 Mic Pre/Instrument) for the price and size. I can not comment on the Saffire-other than it has very similar AD/DA conversion technology(Jet PLL and Dice)-though converters and Preamps are slightly
different. The DSP on the Focusrite would add another hundred bucks over the standard Pro24-which is the same price as the Presonus FS Mobile. I would be surprised if you could hear much difference between these two units. The Focusrite does have ADAT in if you need that but I mainly record line-level hardware keyboards and sound modules/samplers, a bass and a single condensor mic. Both have MIDI and SPIDF.
Having used every DAW known to man-theres alot to like about the included Studio One Artist-a full version of their new software. It has good workflow, its easy on the eyes, and has alot of drag and drop functionality like Tracktion. The only DAW/recording software I use anymore is Reaper and Studio One-despite having learned how to use Cubase, Pro Tools, Logic, DP, Sonar and others. Sonar would be my second choice on a PC-which Im assuming is what youll be using. Logic or Digital Performer are popular choices for the MAC-but even Garage Band is quite good if your just doing basic recording. Not saying any one of these DAWs are better than the others-each has a slightly different feel, look, workflow and features. I think its awesome that my two favorites (Reaper/Studio One) happen to cost very little or free/included with hardware for fully functional versions.
The M-Audios are nicely spec'd/equipped and can use Pro Tools but you'll have to drop an extra couple hundred for the software with a company that has the worst tech support of any audio company Im aware of in history! They actually offered me tech support in the first year bc the MBOX I had purchased was failing...at $1 a minute...it was faulty hardware...lesson learned. Though most people have good luck with the M-Audio units for their stability-I recently had a friend complain of a driver issue that causes his monitors to play at full volume intermittently. And I just cant get over the cheap plastic knobs.
TC Konnekt and Lexicon are some other nice alternatives but I have not tried either personally. The Lexicon is manufactred in the US and bc its USB, should work on any and all computers-but not necessarily bc drivers can be tricky with any interface/OS. Plus it comes with a Lexicon reverb. I think Im talking myself into buying one...maybe for the family's new IMAC. The TC Konnekt 6/24D have their virtues like good monitoring management and TC reverb. Either of these would be high on my list if recording vocals (and/or you need a higher quality reverb) were a priority but since I dont sing and mostly just record bass...
I purchased the Firestudio Mobile and have had to make just a few minor adjustments after contacting tech support and its working very well. Their tech support is very good overall-quick and responsive.
You may also want to seriously consider one of the Line 6 interfaces like the Toneport or Pod X3. I was given a Toneport KB37 recently by a family member and was surprised at how great it sounds...and the Gear Box software is outstanding for guitar/bass. The Presonus comes with similar stuff-Guitar Rig LE. I use the Presonus and the Line 6 side by side daily with no issues. If your using a laptop with mainly just guitar I would definitely go Line 6 bc of the USB and Gear BOX.
The high impedance instrument inputs on all of these are very good for the money. I picked up the Presonus bc of the build quality, software package, and bc I know alot of people that use and like the Pres on their Fire--- line of interfaces. And I really needed those 6 line level inputs on the back! I think theyre all really good choices and it mostly comes down to what features are most important to you. There has been no better time in history than now to do professional recordings with minimal resources.