Well, applications are most certainly a major consideration here ... which includes frequently shot subject matter, type of light frequently shot in, do you favor wider lenses over longer ones, and whether you favor B&W over color?
Keep in mind that the M8 is a crop-frame camera with a lens factor of 1.33X, so a 50mm isn't a 50mm field of view, it's about 67mm. So, if you prefer a 35mm field-of-view for your go-to-lens, then one of the 28mm lenses is where to look. I can recommend the VC 28/1.8 with few reservations ... with one caveat ... it isn't coded for use on the M8.
F/1.8 isn't appreciably different compared to f/2, but the price difference compared to the Leica M 28/2 ASPH can be a major consideration. When you drop below a 50mm focal length on the crop frame M8, hand holding is a little bit easier at slightly slower shutter speeds, so f/1.8 should work fine @ ISO 640 for a majority of conditions.
It is a shame that Leica didn't provide the M8 with more ISO selections like they did with the M9. Jumping from 640 to 1250 is a real working issue with that camera compared to being able to select ISO 800 or 1000.
-Marc