I have both the SL's and the C's from 2-4 years ago.

I have not noticed any durability difference between the 2, except that I have snapped a spindle once on my SL's (new spindles arrived in one day after contacting CB). I have raced them both and have had no issues with either of them.

The SL's are a few grams lighter.

I have rebuilt them both on one occasion with new bushings, bearings and seals.

The SL's wont develop surface rust on the spindle.

The SL's are better looking in my opinion (shiny slender spindle)

They used to have the same pedal body but I don't know if thats the case anymore

I have no experience with the MXRs except looking at them. They look cheap.