M4 feed ramps were developed for - wait for it - M4's. The M16 has always had feed ramps, the M4 with carbine gas cycled so much faster it can literally knock the cartridge forward out of the mag and needed the deeper cuts into the magwell. Rifle length gas and buffers run slower with more inertia and don't bang the case forward as much.
Which is the point: DON'T BUY BRAND, BUY FEATURES. Two pages of BS over which is good or bad isn't the real issue. Specify the features you want, and the brands select themselves. Match the barrel and caliber to the job first, then the makers become readily apparent. Colt isn't in the 6.8SPC biz, so they fell out first thing on my build, I went ARP.
That's not recommending one over the other, it's simple product selection. Had I the choice over again, I would have bought a double minutes card and not given up the previous cell phone - which had an alarm that worked even when it was off. Specify the features you need, let the brand fans duke it out at the bar Saturday nite.
Barrel and caliber, upper, optic, furniture, and trigger, in that order. Price sorts itself out as you move thru the list, and makes some choices painfully obvious. If it's a milspec 5.56 with 16" midlength gas, the A3 upper is still desireable, a 3x9 variable doesn't match up, a sniper stock is wasted money, a free float even more so, and the trigger a feel good item. It's 2MOA - not 1/2 MOA - all those doodads wont' even begin to get it there, it's incapable of shooting better than 2MOA.
If anything, someone insisting one brand over the other is wasting everyone's time and missing the big picture - what's the best features to match what you want to do?
If it's just to blow $1000 and look cool posing at the next gun show, we've seen that with stainless tactical railed Mini 14's, and that guy searched long and hard for those black XXXL BDU's.