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.