Is there any difference apart from weight between the three (951,071 and 991)? Anyone know what the weights are?

951 and 971 are the same but with a coating on the 971- the 991 is hollowpin I believe so will be slightly lighter. I ran 3 chains, swapping every two weeks and noticed no difference between them. The 951 is great value, I don't see the need to spend more for the coating tbh.

951 is std links, 971 has nickel coated outer plates,991 has 3 versions all nickel plated links reg, cross step (double pressed pins for extra strength) and hollow pin for low weight.
I'd still buy the 951 and keep it well lubed to stop it rusting.