I've played both instruments, Piano for an extensive amount of time. I'd say that both are just as easy to play, but I found that sheet music for piano is generally more boring than that of guitar. I found practicing piano much less bearable. Guitar also has tabs, which means you can skate along without learning to read music and still learn to play. While piano is much more difficult than pressing a key, as you have to press many keys simultaneously with one hand while doing the same with another, guitar is also pressing down a string with one finger, hitting that string with the pick in your other hand, and then moving both of your hands to a different string (Or at least moving your hand to a different fret) and hitting a string again. I find that the two instruments are mildly difficult to compare, because they involve totally different means of producing music.
I'd say that it's all based on how your brain works.