I can't tell you which to purchase, I'm partial to the M50. I agree with everyone else, pick the the one you like the best. The best advice I can offer is my experience. I started riding October of 2005 at which time I purchased a Ninja 250 while in the Motorcycle Safty Course. Last March I purchased the Ninja 500. I'm now in the process of trading the 500 in on the M50. Going through the class and see how you feel about riding.
My question is for anyone going from a sportbike to a cruiser, do you regret the change or not? I'm choosing it because I don't like the layover feel of the higher cc sportbikes.