I've used the 951, the 971 and the 991. If I divide the cost by the number of miles I usually get out of a chain, I find that the 991 is worth the extra cost (but not by much).

I usually get about 4000 miles out of a chain, sometimes less. If I was taking a long tour, I'd probably start with a new chain and plan to swap it out after 2000 or 3000 miles just to be extra safe. When I'm home, I use a chain measuring tool and swap out the chain as soon as it has 0.75% elongation. But I'm not going to carry that tool on a tour, even though it is pretty light.

Chain wear varies signicantly by person, depending on how you pedal and how often you lube and clean.