The 350R has speed on it's side(300 vs 280 for M50). It is a beast and half to control though and took me about and hour and a half to get even close to proficient with it. It has a shit-ton of oversteer. You can see that my lines are better with the M50 and I still had a slower time.
Long story short:
M50 is incredible nimble and incredibly easy to fly and feels fast. Really fun. Will beat the 350R in tight tracks with lots of turns (track 3)
350R is real fast and feels like your trying to steer a red rider wagon full of cement with one broken wheel down a hill with two rockets strapped to the back. Really rewarding to fly thoguh. You feel like you've accomplished something the first time you actually finish below 1:20.00. Will beat the M50 in any straight line and in a track with moderate turns (track 1)
Edit:
TL;DR
M50=easy to fly, nimble 350R=hard to fly, fast