When it comes to my personal preference, both are equally important. When it comes to making a successful webcomic though, it's all about the art. Comics with good art will attract attention even if the story is crap. It's not because I think people are shallow-minded or anything negative like that, but it's simply the delivery of the medium being visual.
The art can be judged in a few seconds just by looking at it, but you have to read for a while to judge the story. You can see a banner and immediately, if it has bad art, you won't click on it. But if you see a banner ad that has really good art, you will click on it, and you will read a few pages at least before you decide if you want to continue, or it's not worth reading.
Writing is also more universal, in that everyone thinks that they are good at it. If you have vivid imagination and a word processor, you probably think you are good at writing. With so many "good writers" in the world, it can be difficult to find perspective on where you stand, to evaluate yourself, or to stand out from the crowd. Especially with some of the absolute garbage that ends up banking millions in popular media, while amazing works remain obscure. It's really a crapshoot.
I find it's also easier to see clear forward progress with art. An artist can improve page-to-page and you can immediately see the results. Where as with writing, you may have to wait for years to see if a long-form comic's story ever goes anywhere.
I enjoy art and writing equally, but I never feel like I'm good or bad at either thing. More like, I'll look back on something I did before, and I'll see a few things I did well, but a lot of stuff I could have done better.