If you want a sports car, get a road bike.
Hard tail = dirt track racer.
Full suspension = Baja truck.
In my area, there are a lot of washboard fire-roads that are connectors between trails. Riding a gradual washboard at 15 mph, just beats you up, it's not fun and it's not really avoidable. Modern full suspension bikes can climb really well, and don't beat you up on downhills.