I think there's enough to difference of opinion in this thread to suggest it's impossible to tell.
I was going to say drawing would be harder though, because every person is naturally trained to recognise visual objects and their representations, so most people will be able to tell if a drawing is bad up until quite a high level.
Like I don't know a thing about art but I can realise that whilst it does it's job and the stories great, this http://togm.webcomic.ws/
isn't quite as good as this
http://www.casualvillain.com/Unsounded/comic/ch01/ch01_16.html
(Not the best example, because honestly, the writings probably a little bit better in Unsounded too, I'm not TOGM bashing, it's just Unsounded is genius to the highest levels)
whereas I don't think we're able to seperate good/bad writing quite as easily. More with dialogue but less with the actual prose and story structure. It's still not a huge difference in sensitivity between the two though.
Also stuff like Eragon and Twilight entertains a whole bunch of people, whereas I'm not sure if pictures of the same quality would be as popular. But maybe that's just because I'm a writing guy and I wouldn't be able to even point out what a picture of the same level was
EDIT: Also check out Unsoundedcomic.com

I cheated by showing a particularly good page, but even an average page looks like this and somehow it updates 3 days a week, sometimes with multiple pages and it's just one person? I'm not sure how the lady does it, but I think producing that much aren't must involve some sort of demonic pact