Very interesting decision from the one of the leading scientific publications to publish an article about Reiki therapy.
http://www.nature.com/news/consider-all-the-evidence-on-alternative-therapies-1.18547
[Edit: should be Nature publishes an article about alternative therapy]
Based on what I've encountered, I've interpreted the Japanese version as being more broad and metaphysical (the power of friendship, Killing intent), whereas the Chinese version is more like Alchemy: not quite science, but sorta-kinda tries to be (Fengshui and TCM, but also conservation of energy and such). There is considerable overlap, since ki is literally qi filtered through Japanese culture, but I generally expect people who talk about qi to be more interested in the Alternative Medicine route, whereas Ki indicates one or more of anime fan / Aikido practitioner / practitioner of Japanese spirituality. (These are more probabilities than hard categories; Reiki is a good counterexample.)
Chinese spirituality/meditation practices/internal martial arts like T'ai chi ch'uan and Qigong also use the concept of qi in that sense. In fact, all Japanese martial arts and spirituality/meditation practices derive from Chinese ones.
Qi, ki, prana, etc. are pretty much the same concept, also similar to traditional Western concepts such as pneuma (spirit), psyche... (read more)