My sitbones also measure 150. I got the B17 in the regular width (170 mm) and my fitter (who I saw about 3 weeks after I got the saddle) said it is just barely wide enough for me, and I could go a little wider.
Now that the weather is hot, I can see sweat marks on the saddle when I stop during a ride and they are quite far to the outside. I'd try a B17 "S", which is supposed to be 177 mm wide, but I'm afraid the rails would be too short. The regular B17 is comfortable, though, so I'll stick with it. Someday I may get a second saddle (for my second bike) and maybe I'll try the B17 S. I'll definitely get it from www.wallbike.com, because of their 6-month return policy on Brooks saddles.
I'd recommend going with wallbike, because you can try out a few and send them back if they don't work. Also, if you call and talk to Bill, maybe he can help you decide which model is likely to be the right one.
Good luck! Jenn



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