Turning the power off and right after turning it on is harmfull to the cartridge, and can corrupt data. You need to turn off, wait some seconds and turn it on again. So it is safer and faster to do the soft reset.
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Also, if you turn off the game and turn it back on, it uses a burst of battery power to turn on. Soft-resetting avoids this burst of battery power, so it will help the DS's battery last longer before you need to recharge it.
Speaking of prolonged battery life, I'm pretty sure that the DS's battery depletes slightly faster when it is in "wireless mode" (when the power light is flashing), so keeping the C-Gear off when you don't need it might also help the battery last longer between recharges. (I just thought I'd mention this. XD )