The SEM 21 is a really, really good lens. It's also nice and compact and is fully corrected in-camera, if you plan to shoot it with a digital body. Therefore, no odd corner colorations, vignetting, etc. It also takes 46mm filters which many other Leica lenses use. On the downside, it's a tad on the slow side.
That said, the Zeiss T* Biogon 2,8/25 ZM is also an outstanding lens. It's one of, if not the sharpest ZM lenses. The problem is that it can have odd coloration in the corners and vignetting, depending on the coding applied, which either must be done with a Sharpie marker or performed by a third-party (e.g. DAG). To be fair, once properly coded and with up-to-date camera firmware, it works great. It's a bit faster than the SEM and also a bit larger.
Finally, 21mm is a very common focal length in the Leica world... Not so much 25mm, which is a bit of an oddball. Not a big deal though really, and you'll need an external VF for either one. Personally, I use the Voigtländer 21/25 metal finder and it works awesome.
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