The functional difference is that the MP-2 has the M/S matrix. The mixpre doesn't. The FP24 is basically a rebadged mixpre. There might be more to it than that. But the MP-2 is discontinued, and the FP24 might also be discontinued. Meaning only the mixpre can be had new.
I just bought 2x MM-1's less than two weeks ago. MP-1's on steroids. Ordered MP-1's consecutive serials, got two MM-1's consecutive serials. I wanted more battery life, without using externals. The basic preamp circuitry is supposedly very similar across all of SD's devices. So you might not hear any differences. Spec wise the battery life portion is about all that varies between the MM-1 and MP-1 (20 hours vs. 24 hours / no phantom). The MM-1's having a few extras that the MP-1 doesn't have. Great for resale, not so good for battery life. Meets my needs though, and rather nice sounding compared to my previous kit.
One downside is no stereo monitoring without using something external with 2x MM-1's. Or maybe it is, but the spec sheet doesn't look pretty for that configuration. I don't need that (yet) since the input device (Korg MR-1000) has that facility. The mixpre has monitoring AFAIK (but NO M/S matrix). The mixpre runs on 2x AAs for BOTH channels. 2x MM-1's run on 2x AAs for EACH channel, which is why I went that route.