As someone who's written many of both and had some success with shorts, I'd lean toward the novel. I've seen it written that shorts are harder because that adage about omitting needless words holds more true because you've got to build character, plot, interest in a shorter span. The path for a short might be more clearly defined (i.e., restrictive), but the decorations our left to our imagination (here's where I'm imagining Mel Gibson shouting "Freedom").
In a novel, though the path may be a bit more ambiguous or multi-forked (i.e., open for exploration), we've got to deal with a fairly grumpy HOA. That is, the publishing industry is fully tuned to producing the most perfect (in terms of saleability) novel. Rules, rules, rules (or at least stern recommendations). These can be helpful for finding a path (mundane?), but sometimes it can lead to the artistic gallows.
What do you think?