They perform the same, that is, the rifle works.  But the McFarland seems to be more durable.  The ends of the regular rings tend to break off sometimes (worst with some rifles than others), the McFarland does not suffer this problem.

On the other hand, the McFarland is quite stiff at first, and needs a little break in time.  Regular gas rings are not tight at first, or not to this extent.

Cost for one McFarland averages about $3 each, and for a set of regular gas rings, $1 ea or $3 for a set of three.  So, either way, cost is not a factor.

Most manufacturers still supply their new rifles with the regular gas rings.  But Armalite and a few others now supply their factory rifles with McFarland rings.

Go with what you feel comfortable with.  I have both types on my AR's but am not rushing to change over to all McFarlands.

I have one rifle, perhaps a rough spot in the carrier?, would require replacing the end gas ring every 2 or three trips to the range.  Changing to the McFarland stopped the problem, and I have not changed the McFarland ring since.

I do think, from my usage, that McFarland rings will last longer.

Don't care.  Never had a prob with ANY of the stock springs, rings, or things in my Colt's or Armalites.

You can drive yourself crazy trying to justify these whacky upgrades, and before you know it you have a gun that isn't reliable and you don't know which gizmo is causing it.

I don't like the McFarland rings. Since it's a continuous ring, it doesn't compress much inside the cylinder in     the carrier. The one on my AR10 carbine wore out in about 500 rounds. Sadly there don't appear to be any conventional rings available for AR10's unless you are dumb enough to give Knight's $30 for a set of 3 rings, if you can even get them to sell them to you that is. I'll know after this weekend if the first ring wearing out was a fluke.

Quoted: What are the ill effects of a worn out  gas rings?

Kinda like a worn out car engine - the gasses blow past the rings. In my case it caused short stroking and FTF's and FTE's. Mine was so worn out that if you held the bolt/carrier assembly by the carrier, vertically, with the bolt facing up, the bolt would fall (retract) back into the carrier (hope that makes sense).

ETA - To Armalite's credit, they sent me a new ring and a new extractor for free (they think the extractor was causing the FTE's). Again, I'll know more after I run a few hundred rounds through it this weekend.