Uranium bomb is much easier to build according to what I heard. It boosts their political and military leverage to have large pop(bigger than Hiroshima size) and show outsiders that they do have goods, putting an end to whispering in some corners that N. Korea is faking nuclear tests.
NK doesn't have the beginning of the technology to actually make a uranium bomb and has shown little effort to do so, as compared to the effort they are making to get the enriching going.
NK has had it with Plutonium, both an an energy produce and as a weapon, witness the last two pathetic tests. They legitimately and badly need nuke fuel for energy and are building a light water power plant that uses uranium for fuel.
The big problem for NK is where to buy the uranium ore, China has very little and won't give it up. That leaves Niger and Namibia, maybe Kazakhstan.
There is a working theory that this centrifuge facility is a "Potempkin". A decoy, that they are not really enriching beyond experimental uses. That the real goal is to work the bugs out of Iran and other's chronic breakdown problems with their P-1 and P-2 centrifuges. Then sell the P-1 and P-2 centrifuges and rent out the engineers to set it up. That theory is driven by the mystery of where NK would ever get the low grade uranium to start with. Also, NK needs cash, bad. What else do they have to offer Syria and Venezuela and Myanmar?