If you have alot of power then a stiff racquet would be more beneficial and give you more power. Otherwise, it'd be hard to use a stiff racquet when you're just starting out.
I find stiffer racquets give better control. Suppose someone is smashing at you and you manage to return it with your racquet. If your racquet is very flexible, then the height and distance of your return is highly dependent upon your opponent's smashing speed. This could lead to situations where you either return too high or too short. With a stiffer racquet, the placement will be more dependent upon on fast or slow you are moving your racquet upon contact. Thus, you have greater control over where to return smashes. This holds true for all returns... but is more evident when they hit hard. Not a big difference though. I find increasing the tension on your strings will do the job too.
