general rule of thumb:
Soft = freestyle
Usually softer boots are best for park. The flexibility is a must for park riders, it allows for more for a more forgiving landing off jumps, forgiving on weight distribution on jibs, and tweakability.
Stiff = freeride
Generally a rider whose style is more freeride wants stiffer boots for the response. Having more response on the mountain allows a rider to make those fine tune adjustments while carving and such. This was a fairly general response lol