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If you are not using wide angle lenses, the .85 is usually considered
the best choice for 50mm & telephoto lenses.

As always, the trouble starts because "one size doesn't fit all". Some
people will strongly prefer the greater magnification of the .85 (and
maybe use their left eye to see what's outside the 50mm framelines),
others might prefer to have more space outside the frameline inside the
viewfinder.

Ideal advice:
Try to compare the .85 viewfinder with the others in real life, taking
time to use them extensively in order to get a feel for which fits your
needs best.

Short advice:
Go for the .85, especially if you're getting a good deal on it!

BTW: In case there is still some confusion, the .85 has a *higher*
magnification than the other two, meaning the subject is also bigger.

The best all-round finder is the 0.72X, with 28, 35, 50, 75, 90 and
135mm frames.  The 0.58X drops the 135mm frame, and the 0.85X drops
the 28mm frame.  

A lot will depend on whether you wear glasses.  The eye relief of both
the 0.58X and 0.85X is greater than the 0.72X, so they are better for
spectacle wearers.

In general terms, if you will be using lenses in the standard (50mm)
to telephoto range, you will be better with the 0.85X.  If you will be
using mainly 28mm and 35mm wide angle lenses, you will be better with
the 0.58X.  For everyone else, the 0.72X is the best choice, and the
vast majority of Leica M bodies are sold with this viewfinder.

I strongly suggest that you should try the various viewfinders before
you buy.  Even without a lens fitted, you can change between the frame
pairs using the lever on the front of the body.  There is quite a lot
of difference between the three magnifications, so it should be fairly
easy to decide which one suits you best.

If you have an eye to resale at some point, the 0.72X is the best one
to buy because it will sell more easily.

My own preference is the M7 with 0.72X, even though I wear glasses.  I
use 24mm, 35mm, 50mm and 90mm lenses most of the time.  The 35mm and
90mm frames work well for me, and I use an accessory viewfinder for
the 24mm.  I cannot see the 28mm frame with my glasses on, but I don't
use a 28mm lens on my Leicas so that isn't a problem.  

I also have an M7 with 0.58X but it is not as pleasant to use because
the viewfinder image is smaller.  Finally, I have an M3 which has a
0.91X finder.  I particularly enjoy using this with 90mm and 135mm
telephoto lenses because it has a large, bright image and clear,
accurate frame lines.

The Leica web site used to have a comparison between the various
viewfinders.  I never found it very useful; it is best to try them for
yourself.  Good luck!  Enjoy your purchase.

I think you have it backwards, a 0.85 shows a larger image than a 0.72 which
shows
a larger image than a 0.58.  A 1.0 shows things life-size, an .85 shows
things 15%
smaller.

My M6 has the .072 and I have no problems framing 35-135mm. I do have
to remove my glasses to see the 28mm frame though.
I would go with either if you don't plan to use a 28mm lens..
I tried the 058 and found the low mag too difficult to accurately
focus.