Which would you go with SRAM PG950 vs Shimano XT M770? I currently am running the pg950 on my bike but need to replace it. I can get either one for about the same cost. Being a clyde, I imagine heavy duty over "weight saving". Just not sure if the XT is more heavy duty than the 950 or ?

neither; they are both lightweight cassettes, move down either line for heavier duty components; SRAM PG930 or Shimano HG300 or HG400.

Contrary to this statement, Shimano's most durable cassettes are typically the higher-end. With Shimano cassettes, the thing to look for is how the gear blocks are attached. Lower end cassettes typically have individual gears with maybe the top 2 gear on a common block. Higher-end cassettes will typically have most of the gears bonded to either one or two blocks. Using a common block distributes the torque over a large area, and prevents the splines from digging into the hub.

In the OP's particular case:
The XT770 cassette and HG400 have very similar block designs, and should have similar durability. If you want to save weight XT770, money then HG400.

I have never seen any reliable tests indicating expensive cassettes to be any more durable than the cheaper ones. The cheaper cassettes is usually one big block and are less prone to wear aluminium hubs.