I'm a fan of canes. Nothing beats the quick, crisp SMACK that they deliver, and they are long enough to be easy to wield. The naughty ruler looks very cool, but the short length really limits the positions you can use. Generally with something this short, you're limiting yourself to OTK (over-the-knee) spanking.
you SHOULD get both since they have completely different effects on your subject BUT since the ruler is sold out atm i think your decision is pretty easy