This is the IO port of ATMEGA328p according to 76 page of datasheet.
As I know, Pull up is the most suitable system because lots of voltage spikes are logic 1.So, if we can give logic 1 normally, we can avoid unwanted triggering causes by spikes.
This thread has some explanation. But,it can't satisfy me.because ATMEGA328p port is bi-directional.
Explain what is the most suitable way(pull up/pull down) when we use a button input to the port with help of diagram above.