I'm not much of a tech head, so the guts of these units aren't my specialty. I can only comment on the things like user interface and, of course, the bottom line like sound output.
Mini-MP (Contrary to a previous commenters' input, it seems that Apogee has indeed discontinued production sometime earlier this year...seems like I read that they stopped last March or April. A check of the website shows that the mini-MX series is no longer part of their active line...see
www.apogeedigital.com.)
Pros
Great warmth, smooth sound over entire range of frequecies, one of the warmest pre-amps available out of the box, runs on any voltage battery, fairly low current draw (runs quite a long time on a 9V wally world), reasonable metering, great value to price ratio (at $500 to $600 used, one of the great values in preamps), has an M/S setting although I never used it so can't really comment, some other miscellaneous controls that one might find useful but I never used for live recording. Cosmetically, if this is important to you, the lights on the Mini-MP are very attractive and I personally love the purple base colors.
Cons
Inconvenient form factor with the long, skinny design and the hardware on the end, if you swap polarity it's dead ($100 factory fix) but shouldn't have a reason to swap polarity (I did it by accident when I was trying to diagnose a problem and thought polarity might be an issue...my bad and I learned a good lesson.), external powering required. Connectors come out the back, meaning when you set the unit up in your bag it's resting on the cables.
Mix-Pre Pros
Excellent metering, headphone amp, on-board powering with 2 AA batteries, small size footprint, built for suviving nuclear war, SD is one of the best customer service companys in the business. Connectors come out the side.
Cons
Two AAs will only last between 3 and 4 hours...enough for a show but nothing longer, meter lights...even when dimmed are very bright, if it's like the MP-2, the gain controls are VERY sensitive and you can get 3 - 5 db output change with minor touching of the knobs
Unfortunately, there aren't a whole lot of mix-pre samples out on Live Music Archive. However, the mix-pre has been stated to be tigher sounding than the MP-2. That would be good, but subjectively stated, the mix-pre would have to be tightened up A LOT from the MP-2 before it matched the creamy awesome sound of the Mini-MP...but again that's my subjective opinion based on the MP-2. I think that there's plenty of people out there in preamp-land that like the MP-2 sound just fine...try as I have, unfortunately I'm just not one of them.