NG is their top of the line. Features exclusive to it are:
- usb rechargeable battery
- pedal smoothness data
- torque data
I find all three of the above, especially the latter two to be of questionable value. While a USB rechargeable battery sounds great, it is one more thing to forget to charge (i.e.,Di2 or eTap). I have had so many friends forget to charge their stuff 
Also, p2max battery changes take a couple min and you only need to do it a couple times a year.
I would recommend going w/the NGeco if you do the p2max.