When I first decided I was going to write (and finish) a novel, the first idea that came to mind was a sci-fi romance called Heart of Steel. That was fine as far as it went, until the time came to do a little genre research, whereupon I found the relative handful of sci-fi romance titles were tossed into the same section as paranormal romance. That struck me as odd, since sci-fi and paranormal are only similar in that they're both spec fiction. Aside from that, they're worlds apart. Now, I know that the people in charge of cataloging the romance subgenres are going to keep using the terms interchangeably while sci-fi romance is even nichier than paranormal romance, but I thought I'd try to distinguish the two.
First, let's talk about setting. Paranormal romances tend to be set in roughly the modern day, plus or minus twenty years or so. They're usually like "our" Earth, except for the presence of the paranormal stuff, which may or may not protect itself with a conspiracy to keep normal folks from discovering (and freaking out over) it. This offers the reader a fairly comfortable frame of reference, and allows for the possibility of an everyman or everywoman lead. Sci-fi romance can be set anywhere from 100 years ago (in the case of steampunk) to several hundred years in the future (for nearly everything else). It also doesn't have to be set on Earth, let alone "our" Earth--I've seen titles set on alien planets as often as a futuristic Earth that might be a mind-boggling dystopia, a post-apocalyptic hell, or a wondrous speculation of miracles yet to come.
Now let's talk about the critters. Paranormal romances tend more towards magical or supernatural creatures, including but not limited to vampires, spellcasters, ghosts, demons, angels, and shapeshifters. Any one of these may be a love interest (vampires seem to be popular these days), and they can be either gender. Sci-fi romances can have aliens (usually humanoid), cyborgs, robots(!), and any number of metahumans with superpowers not otherwise covered under the paranormal umbrella. Again, any one of these can be the love interest in your story (yes, even the robots), but while they can be either gender, there seems to be a slight preference for males, regardless of the gender of the other lead.
Finally, let's talk about the heroes. Paranormal romances tend towards intuitive heroes who aren't afraid to resort to physical combat to get the job done. They use their street-smarts, or instincts, or gut feelings to get out of a scrape. Conversely, sci-fi romances tend to have intellectual heroes who prefer to think their way out of a scrape. They might or might not be able to build a death ray out of some aluminum foil and a car battery, but they're more likely to try to make a plan rather than punch, kick, or bite (depending on preference) their way through obstacles. This is not to say that all PNR heroes are brutish beasts (even if other members of their species are) or that SFR heroes are all emotionless robots (even if they literally are). Even a steampunk artificer might know how to throw a solid punch in a pinch, but if he has time and the death ray option is open, well, he's going to build himself a freaking death ray.
Now, I know that there's going to be some overlap between SFR and PNR stories, especially if the author takes the science fantasy route or the Magic Is Sufficiently Advanced Technology route, but these are the distinctions I've seen. Unfortunately, the fact that there aren't separate categories for each means that someone looking for one or the other might have to hunt around in Romance or Science Fiction to get your speculative romance on, regardless of which type you prefer. Fortunately, with the increase of self-published authors, I imagine that both of these categories may increase in size, meaning that the cataloging people in bookstores (online and physical) may eventually separate the two properly. In the meantime, though, happy hunting.