When we moved up we went with a full sized bed. It's so much nicer when they lay there to do homework or read. And having sleepovers is easier b/c there is room for the guest child already.
For my son, we did the full sized bed also. He pretty much started out with it (age 3) and it was very nice for us to lie on at night to read bedtime stories (3 of us, me, him and his younger sister)... we all had room to lie down/prop up together with the books. But, when it came to sleep overs, boys tend to sleep on the floor in sleeping bags more than girls, it seems.
It really depends on your space. I know you have bedding sets already, but she might be ready for something a bit more mature? I don't know. 7 is still pretty young. We upgraded my daughter's bed when she was closer to age 10. So she was ready for a slightly more "mature" style. It's also more difficult to find "kiddie" prints in full size than in twin. So, if she is wanting (and you are willing) to put unicorns and cupcakes on her full sized bed, realize that they will be more difficult to find (and more expensive).
I didn't find that full sized mattress sets cost any more than twins, really. Just for the record. As my son got older (high school) he actually wanted to switch to a twin bed. He wanted to have the space in his room more than he wanted the space to lie on, on his bed. Daughter has ALWAYS been happy with her full sized. She is 15 now.
What does your daughter think? Not that you have to do what she wants, but it might be worth at least asking her thoughts on it. You never know. She might prefer to stay in a twin. I had a difficult time finding a twin that didn't appear too juvenile for what I wanted to do. :/