It really doesn't matter until you get into the 16" range with a gun that cycles VERY fast.
The whole H buffer thing was to slow the bolt down so the carrier does not outrun the magazine spring......if it does....the carrier wants to strip a round before the mag spring has fully pushed the next round up into full position. Maintain your rifle and it's a non issue.
I have both and they all run 100% in semi auto variants. Now....Del Ton and others offer M4 ramps for about $25 and that's a nice upgrade for the risk of bad mag springs.
http://www.ar15barrels.com/gfx/feedramps.jpg
Not totally necessary to make an AR run.......not worth high dollar.....nice to have on a full out AR you intend to abuse.
Lycanreallyyourchoicethrope