Yeah, it's kind of silly that the button combination for soft reset is exactly the same as that for screenshot--it just depends on how long you hold the buttons down for. Holding them down for about 2 seconds gives you a screenshot; holding them down for 10-15 seconds soft resets.
I think another difference is that to take a screenshot, you really have to press the power and volume down buttons at the exact same time, whereas to soft reset, it doesn't have to be at the exact same time.