I take both, but if I was only traveling with one it would be the M2P for sure. I have 1840 miles in 351 flights on one of my M2Ps now and it's been 100% reliable.
I just got it late today and only had the chance to register and get in a 10 minute flight. My flight is Tuesday and I don’t think I’ll get in enough flight time, plus I only have the one battery.