HN. Splitting off a sliver of a community isn't necessarily viable, but that sliver is alive and well on HN and posts about design that are interesting tend to get upvoted, but not a whole lot of those posts are made. It takes effort and that's in part why reddit is so successful, there is enough traffic overall that niches can flourish that would never succeed as a stand-alone website.

HN solves this problem by not having categories, which is another valid path. But to make a 'HN for designers' is DOA unless you are willing to put in a lot of time, effort and probably money to attract and retain an audience.