Ok, in your situation I would go for an M4P or M6 "classic" which have the 0.72x viewfinder. Generally, the 0.85x viewfinder is intended for fast or longer lenses, the 0.58x for wide angle users.
This M4P is identical to the M6 "classic" except without the lightmeter.
With contacts you will easily be able to see the 28mm frame in this viewfinder, and will provide good (though not absolutely ideal) magnification for focussing a slower 90mm lens.
Some (non glasses wearing) users use the outside edge of the 0.72x viewfinder to provide a frame for the 24mm lens but this will give you no parallax correction and is not too accurate.
Helen has already given you a great outline of the features of each of the models, so I shall not repeat.
I recommend you go and play with the camera before you buy. I for one find that the R/F camera does not suit me for most of my work, although I have owned an M6 "classic" in the past and still own an M3!