I once read somewhere that you used sci-fi when you dealt with nuts and bolts; fantasy if it was anything else. ;^)
However, I think you're going to be best calling yours Sci-Fi. Urban Fantasy isn't really the right moniker for your genre. Urban Fantasy:
a sub-genre of fantasy defined by place; the fantastic narrative has an urban setting. Many urban fantasies are set in contemporary times and contain supernatural elements. However, the stories can take place in historical, modern, or futuristic periods. The prerequisite is that they must be primarily set in a city. (see: http://en.wikipedia.org/wiki/Urban_fantasy
a genre of fiction that commonly uses magic and other supernatural phenomena as a primary element of plot, theme, or setting. Many works within the genre take place in imaginary worlds where magic is common. Fantasy is generally distinguished from the genres of science fiction and horror by the expectation that it steers clear of (pseudo-)scientific and macabre themes, respectively, though there is a great deal of overlap between the three, all of which are subgenres of speculative fiction. (see: http://en.wikipedia.org/wiki/Fantasy
is a genre of fiction dealing with imaginary but more or less plausible (or at least non-supernatural) content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities. Exploring the consequences of scientific innovations is one purpose of science fiction, making it a "literature of ideas". (see: http://en.wikipedia.org/wiki/Science_fiction
I have a feeling that saying SciFi with romantic elements is where you're going to wind up going. Just remember if you do decide to do anything along the lines of paranormal romance you're really going to have to sell it to make it different from everything else. Good luck!