As concisely as possible: IPFS is a way to publish your data in the cloud (and the related Filecoin project is a way to host your cloud data), urbit is a way to publish and host your identity, computing and data.
As a more specialized project, you'd expect IPFS to be better at the part of the problem it solves, for the same reason a sprinter sprints faster than a decathlete. (Not to mention that JB is awesome.) On the other hand, not having an identity model other than public keys (or, to put it differently, not trying to square Zooko's triangle), imposes certain problems on IPFS that urbit doesn't have. For instance, with routable identities, you don't need a DHT, and so the idea of a hash-addressed namespace is less interesting.
That said, it would be easy to imagine a world in which urbit either could talk to IPFS, or even layered its own filesystem (which has a fairly ordinary git structure under the hood) over IPFS. Like I said, it's a cool project.
