Both have multiple formulae which seem to be a bit different. OS Endurance stays liquid longer, but is less effective at stopping. Stan's has the new(?) "Race" sealant, which is more expensive but has 2x the particulate.
Overall, I prefer original OS as the best all around sealant. BUT, I do find that Stan's is better for mounting tires for the exact same reason it's less good for sealing - it's thinner. Stan's will work its way into smaller nooks and crannies.
The preferred CX recipe for a long time was Stans+Slime *AUTOMOTIVE*. The idea was to get the bigger particle size of Slime Auto with the flow of Stan's. I ran this combo for a long time in my latex tubes but never had occasion, thankfully, to test it. I eventually switched to Stan's only, which was insufficient at 90psi+. Then I switched to Orange Seal only, which is very workable at 90psi+ and saved me having to change tubes a number of times in training. Never had a chance to test it on race day.
But currently I'd say I think a blend of OS *and* Stan's seems ideal. Or, more specifically, mount tires with Stan's and then add OS to seal punctures.
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