corado- I've had both. i hated the 421b. with as many motors as I've tuned i will say that was the worst of all of them. the m2k is a very hot mill, but i think the b5n (modded) will smoke it hands down no matter how hot you make it run. yes the b5n is a variant of the p5 but so is the m2k. i guess the cutting work is more accurate with the b5n? RB Mods is the only place i will send a mill for work and if you've seen his work then I'm sure you would feel the same.
nitro 21- id say $420 for a jp fx b5n is the best price I've seen. the one I'm looking at is said to be around $300-350 but it isnt modded by jp. it is the stock b5n which i will have modded by my guy. he does some very good work and gives more benifits to the motor. longer run times, more power all over the rpm range, cooler running temps, more rpm and longer engine life, not to mention the best customer service to be found.