Well, 10mm is about 1/2", so I would say for the average person, you won't really notice it. However, if you are an avid performance rider, who does alot of climbing and/or sprints when racing, you will notice a slight differance. The idea behind shorter stays is to give a more complete transfer of power quickly for acceleration. Racers usually require this.
However, longer stays will provide you with more impact absorbtion, for greater comfort and stability on very long rides. If you are a touring maniac who loves to do centuries, but is not in racing mode, then the longer stays are the ticket. It just depends on how you want to ride.