Okay, I'll take a stab at answering your question since I do have both of those finders.
The 28mm finder is big and bright. Makes "quick framing" a breeze. So it is great for scale-focusing street photography type shooting. The 28/35 minifinder is small and therefore wonderfully discreet. Makes fitting the camera into a tight bag a much easier proposition. And while I have no problem framing a shot with the minifinder, the 28mm finder is definitely bigger and brighter. Note also that I have not yet tried using the minifinder in a low light situation so can't comment on how much difference in the relative brightness of these two finders might make. Not sure I'm really telling you anything that you didn't already know. The problem is that comparing these two finders is a bit of an apples-to-oranges comparison. They each have their advantages and disadvantages. But they both work well and I do not have a problem using either one of them. So, my recommendation is to get the one that best serves the purposes to which you intend using it.
-Randy
P.S. As Roland (ferider) notes above, the minifinder looks great on an M3. The size, shape and coloring make it look like it was made for it.