On tour I play a 6.5" Black Beauty and my b/u is a 6.5" Supra (both 2006.) I didn't want to 'waste' a BB by having it sit as a spare, and in the single emergency I've had, the Supra was just fine until the BB was back in action (only needed to replace snare wire cord.)
In town, I use various snares, mostly Ludwig and mostly 6.5" metal shells. I don't bring a backup, but I do keep a spare set of snare & tom batters. I used to carry extra cord, felts, and other supplies, but never got around to needing any of them. Maybe I've just been lucky, but in 40+ years of local gigging, I've never had a snare (or pedal or much else) fail. Can't recall ever breaking a head on the job, but that seems more likely than anything else, so at least I'm prepared for that!
Bermuda