There are a bunch of variables that affect the system, and reciprocating mass is one of them. Magazine spring strength, gas port location and size, bolt carrier group being in spec or not, etc , all play into the system and make the AR-15/M4 work or not work.
There are several physical facts to consider:
1 with everything else the same, more gas pressure will cause the BCG to cycle faster
2 with everything else the same, the shorter the gas system, the higher chamber pressure will be when extraction starts
3 with everything else the same, the more reciprocating mass, the longer extraction from #2 will be delayed
4 weak mag springs cause the rounds to rise slower, and are a leading cause of AR-15 malfunctions when the bolt cycles faster than the top round can rise. this failure mode can be reduced by making the bolt carrier group take less time to get back to that portion of the return stroke
5 everything else the same, the more reciprocating mass in the system, the longer the action will take to cycle
6 everything else the same, the longer mass is in motion the more delay until the gun is ready to fire again (and the sight picture stops moving from inertia events)
There are competing theories on what "reduces recoil" the most. I, and just about all competitive 3-Gun / practical rifle shooters, have concluded that less mass produces less sight picture movement and the gun is back on target and ready to fire again sooner. Take that for what it's worth.
Facts #2 and #3 above contribute to the reliability problems in the M4 (and carbine-gas 16"-ers) that caused the SOPMOD/M4 improvement project some years ago. This effort yielded some changes in extractor setup, some bolt improvements, and I think some changes in buffer.
The issues are also covered in this ArmaLite tech note:
http://www.armalite.com/library/techNotes/tnote48.htm
I avoid the M4 gas system (carbine gas) if I can. It has too much gas pressure and the chamber pressure has not dropped enough by the time the bolt starts to unlock. This contributes to reduced reliability and reduces parts lifetime. That said, my 12" SBR has the carbine/M4 gas system and it runs 100% (Noveske upper). I run a regular carbine buffer in this, but a M16 bolt carrier which is a little heavier than a regular AR-15 BC (and it's what was shipped with the upper).
The midlength is a better choice if it can work with your barrel. I run either stock AR-15 or slightly lighter bolt carriers in these and a regular carbine buffer.
With a rifle-length gas system, I again run a regular carbine buffer but then a slightly lightened bolt carrier.