I'd consider the long-term effect of build quality with the ZM as well. Supposedly fixed in newer versions, but consider "the wobble." Especially if buying used.
The 28mm focal length is nice if you have a 50. Much wider than a 35 - enough to be a different experience. A 21 is again a different experience entirely. Between the three, a very nice spread. The wider you go, the more dramatic every 1mm in focal length is.
I never shot with a 28mm on the Ms until I picked up the Cron, and before I knew it, it became one of my favorite focal lengths. You can shoot all day with it. A nice feature is that the framelines are comfortable to work with and easy to see. Of course, you can just use the whole VF with the 25. Somewhat inaccurate, but close enough.
With a 21, you really want a dedicated external VF (or EVF) for best use. Something to consider. But there's something about having such an ultra-wide. It's a whole lot of fun, and great for street if you're the type to get in there with your subjects. The DoF alone makes for a snapshot camera when set to hyperfocal. Unlike a 28, it's not an all-day shooter.
The 25 is... Yes, a bit in between. Optically, it's the best ZM (though they're all really good, especially the Planar). Not as obvious a wide as the 21, but a bit more dramatic than a 28.
Finally, there's the issue of coding. You'll need to deal with it, either temporarily or permanently. So keep that in mind also.
This might help: