The CV 21 versus CV 25 should really come down to how much you want your lens to scream WIDEANGLE.
I found with the 21mm, the images always looked like they were shot with a wide angle. Personally, I'm not a fan of that. I like being able to shoot with lenses that become ambiguous in terms of focal length.
I sold the CV 21mm LTM and picked up the CV 25mm M-mount. It fits my specific wants much more. I can shoot with it in a way that produces images with a wide field of view without punching you in your face about it.
Both lenses are absolutely stellar as far as optics go. You won't complain about either. The LTM 21mm typically comes with a VF as it was originally sold with one. The M-Mount 21mm typically doesn't come with a VF as it was always sold separately. The LTM 25mm isn't rangefinder coupled and instead has click stopped focus points (1m, 1.5m, and 3m IIRC) just so you're aware.
So if the 'wide-angle' look doesn't bother you, I think a 21/35 combo makes sense. I'd recommend finding the LTM version as, personally, I find the focus knob better than the focus tabs the M-mounts have. Plus...you'll probably get a VF with it.
I originally bought the 21mm to compliment my CV 35/2.5. It made sense. However, my kit slowly changed and the CV 40/1.4 became my main lens (having a F4 and F2.5 lens was a bit too slow of a kit). I ditched the 21 and got the 25, giving me a nice 25/40 kit.
Pic related: CV 21mm