As a maker of comics, I actually do both, and for me, writing has always been easier. Now, I think that for the most part, that's a personal thing - I've been writing for a long time, and coming up with stories isn't the problem so much as deciding which stories NOT to tell. Drawing, on the other hand, is something which I added more recently.
In a more general sense, though, I think that drawing is harder. Any fool can write "three guys sat on a bench". But when you're drawing, you have to design the outfits and unique character designs for the the three guys, and you have to make the bench big enough for them to sit on, and you have to draw everything in proportion, and you'll probably have to draw the bench in perspective, and if you're any good you'll add a background, so you have to decide: are they at the park? The beach? The inner city?