I have both a US L2500 and a US M2500, and have had Tribute versions of both the L2K and the M2k. Despite what you may have seen/heard in Ed Friedland's demo where he tried to make them sound similar, in my experience they're very different beasts.
The L—especially with the k-mod for single coil function—is an absolute Swiss Army Knife capable of a huge range of tones, both by flipping the various switches, and by tweaking the two treble/bass cut knobs—*and* the volume knob (as noted above). It can be super-aggressive or quite sweet depending on approach and really rewards time spent getting to know all it can do, especially in a live setting. It also can be run passive or active (a curious sort of active, really) and for many of us it works best as a fully passive bass. It's absolutely one of a kind.
The M on the other hand is a sort of "modern" always-active bass with slightly more house-trained MFD's and a really excellent preamp matched to them. It has a certain kernel tone that characterizes it and it doesn't run too far either side of that basic tone, although the pickup blend is a very useful tool for that purpose.
What I'm writing here makes it seem that the M is something of a less-capable city mouse to the L's burly and multi-faceted country mouse. While that's sort of true, I absolutely *adore* my M2500 and if I had to keep one of them, it might well be the M. Given that I play in a lot of old guy jazz, blues, and music theatre stuff, when I bring the L along I often feel like I've brought a gun to a knife fight. The M always seems to fit in, yet its distinctive voice is obvious in the mix.
Oh, I can keep both? Excellent!