The main difference between a soft-mount and a hard-mount washer is the extraction speed.
Softmount washer-extractors have a higher extraction speed, which deliver a dryer result at the end of the cycle.
Because of the built-in shock absorbers, they can be used at any level of your building, and are more flexible in installation
Hardmount washer-extractors have a lower extraction speed, as they are differently constructed.
Since they need to be anchored to the concrete base in the floor, you can only use them on the ground floor or in the basement.
They mostly have a more favorable purchase price vs the highly featured soft-mount.