So far, I haven't had a chance to do a side-by-side comparison for recordings, but the biggest difference in day-to-day use is having to move from the MixControl software to Focusrite Control for input/output routing. So far, I really like the UI of the Focusrite Control over MixControl.
The 1st Gen is great and it seems like they made smart and practical upgrades for the 3rd Gen (push buttons/knobs/meters). If I didn't get a ridiculously good deal on the new unit, I would have happily kept using the 1st Gen.