Irrelevant unless they're made to the same specs.

Which they're not.

I've used about half a dozen brands over the last 2 years. 4 or 5 of them were manufactured by Kenda (Specialized, Giant, Sunlite and some other brand[s]), and only 2 of them were worth their weight in discarded ragged butyl rubber. Specialized and Giant. Bell makes decent but heavy tubes. I don't care about heavy, and their main selling point was that I can pick them up from WalMart at 2AM.