AVR: Pullup vs Pulldown on button input?
Datasheet shows that 


the noise margin is a little wider for the High logic input state.
input current is microamperes, no serious difference between Hi and Lo states.


Obviously the same noise field cause more voltage to wire+pull-up resistor than to wire + to GND conducting switch. Conclusion: Use pull-up resistors.

This is true of course when only external resistors are compared. If you compare internal pull-up vs external pull-down, of course the external pull- down resistor reduces noise, because it must be quite small to win the internal pull-up. This can make someone to think that the question was originally a trick.

NOTE: In severe conditions, for ex. near radio transmitters (including wlan and phones) the difference is nonexistent, serious design effort is needed to filter the noises out of the signals.