Honestly, given that the rear brake isn't of great use anyway except when you're on an unstable surface or just want to generally slow down a bit (not stop quickly), ISTM that a coaster brake on the back would be ideal. I'm going to go with a coaster on the back, a Nexus 8, and front disc when I finally build my perfect commuter.
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Work: the 8 hours that separates bike rides.