I have both a Colt lightweight A2 upper and a Bushmaster Superlight A3 upper.
AFAIK they are exactly the same diameter. Of course, I could get off my ass and go measure them [:D], but I'm comfortable enough right where I am (and the History Channel is doing a "Bullets" episode of Modern Marvels).
I'm also comfortable saying that they are the same diameter. Or at least close enough for government work.
I have found that they are comperable in terms of accuracy. They should also be equals in durability.
Barring a serious single purpose tackdriver, I'd go with a LW bbl. But that's just me.
FWIW, I'm using the BM SL upper on my primary rifle. The Colt upper is waiting to get reunited with it's factory lower after the Krime Bill dies.
I have also been able to beat the every living crap out of more than a few taxpayer supplied LW bbl'd M16A1's. The only problem I found was trying to qualify after using my rifle (not a rubber duck) for a weekend on a bayonet course. The rifle still functioned perfectly, though the zero was badly off.
I'm sold on LW bbl's.