If you mostly wear contacts, and the main focal length you use is 35mm, then I would advise a 0.72 viewfinder magnification. W/o glasses the 35mm framelines are very easy to see. With glasses, you will probably need to rotate your eye somewhat to see the entire 35mm frameline. With a higher magnification (0.85), the 35mm frame lines are more difficult to see. With a reduced magnification (0.58), the 35mm framelines are very easy to see and there is space around the framelines, which aides in composition. But the image in the reduced magnified viewfinder is very small, which I believe makes focusing more difficult. Also, the framellines for the longer lenses in the 0.58 are very small.
For eyeglass wearers, the Leica viewfinder options are not an ideal. Each one will have pros, cons and compromises.
The other option is to wait until August when the Zeiss Ikon comes out. It has a HUGE viewfinder (with 0.74 mag) which, I believe, will make the 0.58 M somewhat less desirable. Eyeglass wearers who have viewed the prototype have reported no problems in seeing the 35mm framelines in this new camera. But at this point, we don't know how the camera will operate in the field. I suspect there won't be much problems, because the only electronics are a meter and AE, but the Leica M is a known quantity -- it works.