I own a ZM and M7 (revised finder), here is my 2 cents summary.
The viewfinder of the ZM is as good as the M7 if not better (I think I prefer the ZM, also because of its greater base lenght of 55.5mm vs. 49.86mm for the M7@0.72). The ZM finder doesn't flare, contrary to Leica finder, even the revised MP finder which I have on my M7 isn't as good. The ZM has a moving patch, the Leica has a moving frame (when focusing). The framelines of the ZM miss the 75mm and 135mm, but have 28mm and 85mm frames, which the M4-2 do not have. The top speed of the ZM is 1/2000, the Leica 1/1000 only. The ZM shutter is more accurate (electronic), but won't work without batteries (cheap LR/SR 44). The build quality is slightly better with the Leica's : the mount is tighter, the horizontal alignment between viewfinder frame and patch won't move (something common with the ZM is that the patch can be slightly twisted). As concerns the servicing, the Leica is a safer bet, anyone services them, not the case with the Zeiss. The ZM is lighter than the Leica (M7), it feels easier to handle (especially with a Noctilux on it), but does not feel as solid, despite it probably is as much.
Exposure-wise, the ZM is very accurate, large center-weighted measuring area.
For glass wearers, the ZM is more friendly : you can replace the standard eyepiece with any other cheap and quality Nikon or Voigtlaender eyepiece.
All in all, I'd stay with the ZM, even so I mostly use it in manual as it's my habit, I think that it's holding its value pretty well and anyone else in the family can use it - not the case with a non-metered camera -.