The 1091 is basically a little heavier and little more durable the the 1091R. It weighs about 10 grams more and does not have the Lightening Hole in the plate. For ease of install and removal SRAM includes the Power Lock.
The SRAM PC 1091 is a strong, reasonably light chain that gives precise quiet shifting for 10 speed systems.
I have broken many chains in the past and will use nothing but SRAM. The locking mechanism is the best by far and allows easy removal and replacement over and over again without compromising strength. Very light and strong chain, you won't be disappointed.
I won't run anything less than 1091 on my bike. I've broke rival company chains and even a cpl of 9spd ones by SRAM but this has withstood the power I put down and shifts buttery smooth. May try 1091R but not until this wears out. Practically indestructible!