hey dub1h, I recently had to break up with someone who wanted a more time-consuming and emotionally intimate relationship that I could handle. I could be just projecting my experience on your situation, but it sounds to me like you might be walking into the same trap, with you being the one who wants "more" than the other person can give.
First thing to remember is that when poly folks talk about love it's LOVE a.k.a. Romantic Love or Passionate Love, the kind of love where you're euphoric one day and in despair the next (see Roller Coaster thread). This is not the type of love you have for the world, your neighbor, or your family. I wouldn't tell this girl that you love her unless you are "in love with" her, because that's what she's probably going to assume if you say "I love you." And usually one of the unwritten, or sometimes explicitly stated, rules of Friends With Benefits arrangement is that you don't fall in love, you don't get that bond and dependence that leads to the emotional roller coaster. Usually people looking for FWB are explicitly saying they don't want a Relationships and all the pleasures and drama that comes with that, whereas poly is all about Relationships.
It's not just semantics, or different terms for the same thing, it seems to me. The FWB relationship is not intended or expected to progress to anything other than a casual friend with whom one has casual sex, while poly dating can lead to something more serious, with the two (or more) people moving in together, sharing resources, starting a family, etc. as you can see from some of the amazing success stories on this site.
So if you're falling for this girl, or want a girlfriend-type relationship with her, you owe it to her to either tell her, or cut ties with her, since it sounds like she made it pretty clear that's she's looking for something different. Just my opinion, based on what you've told us.