The Center for Strategic and International Studies' Victor Cha and Ellen Kim explain why it matters whether North Korea tested a uranium-based weapon or a plutonium bomb:
A uranium-fueled test would suggest several disturbing new problems in the effort to denuclearize North Korea. First, it would mean that the DPRK has not one, but two ways to make a bomb which doubles the problem. Second, highly-enriched uranium is much easier to hide than plutonium. It can be made in from centrifuges operating in buildings the size of a warehouse unlike the big and easily identifiable footprint of a plutonium nuclear plant facility. Third, the North can potentially produce a lot more uranium than it can plutonium and proliferate horizontally to others (like Iran) who may not need to test a device and feel confident that it has acquired a working device. Moreover, if this is proven to be a test of a miniaturized device as the North claims, then they will have crossed another technological threshold in mating a nuclear warhead with a long-range ballistic missile that could threaten U.S. security and that of its allies. Basically, none of this is good at all.