Assuming I remember correctly

951 is their basic chain, nothing special but still works well, the one on my xtracycle is over 2k miles with plenty of life left.

971 has a nickel coating that makes the chain look much more polished and supposedly helps a little with rust prevention

991 has two models, one has hollow pins to save a little weight, one is their cross step pin which is supposed to be a stronger riveting method. This chain also works well, I got a lot of use out of the one on my mountain bike but I really doubt it is worth the cost over the other two.