Are lighter chains worth the cost? I'm replacing chains on a number of bicycles, and trying to decide what chains to purchase.  I'm a fan of Sram's master links, so I'm comparing the Sram PC-971 to the PC-991.  The PC-991 is considerably more expensive, but it seems like the lighter weight chain would make a significant difference in pedaling performance, because of the angular momentum involved.  Are my instincts correct, or is this just a sales gimmick on Sram's part?  

Also, I've seen some reviews indicating that the hollow-link chains are prone to breakage -- is this something I should be concerned about?
I'm no racer so I'm probably speaking from a very different point of view than you were interested in. I have used Sram 9-speed chains for years on my touring/commuting bicycles. I have used the 971, 981, and 991 and haven't been able to see any difference in performance or longevity, either. With my wet-weather commuting here in the Seattle area where it's pretty nearly wet every day from October through March, and then there's plenty of rain in April and May, too, I have been able to wear out chains and cassettes very quickly.

I'm not a mountain biker--or at least not an avid one--so I have never really hammered the chains in the muck but I have never had one of their chains break. I've just managed to wear them out over time.

I do greatly appreciate the master link because it makes it easy to take the chain off the bike and put it back on again. Before I switched to a bike with a Rohloff internally geared hub for my commuting, I would keep two chains in rotation on my bike and take one off, soak it in cleaner, and let it dry while I put the other clean chain on my bike.