[Bryan]

Simply put its should be a combination of both design and content/seo. If you're looking for someone to put a % on the importance my personal view would be...

Design - 30-35%
Content/SEO - 65-70%

A perfect example is the iPod. There's tons of mp3 players out there that all do the exact same thing. They allow you to add music to them and then search and play music and now sometimes video. Again, many players including Microsofts Zune does this, but then why does Apple hold 75% of the market? It's because of design, layout and simplicity oh and also marketing which also plays into design. Not because of what the product holds as they all pretty much hold the same music.

Same goes for web design. Sure there's exceptions like Craigslist, but in general users would much prefer a website or UI that's easy to look at, well thought out in terms of layout and most importantly easy to find the proper info.

So again, I think it's a combination of the two. Great content and proper seo tatics are obviously more important, but if you can combine that with a beautiful, though out design then you'll probably be even more of a hit.

Bryan