I've had both and this is how I view them:

RCN:
pros = better/more intelligent tech support, TCM and GSN included in basic package (key if you like old timey movies and old timey game shows!), nicer/more polite customer service, fewer wifi outages
cons = the free on-demand selection stinks - very little is available, the on-demand system in general is really clunky and not user-friendly

Comcast:
pros = on-demand system is awesome - lots of free shows and movies (CBS, A&E, Bravo, etc.) available in an easy-to-navigate system
cons = fewer channels in the basic package, tech support is maddeningly unhelpful