Well... I'd go with the legacy. you're only 15yo, and gonna grow more.
Foot seems to grow up for a 2-3 years definately. I don't remember my foot size in 15yrs, but i wear us13 from 18 till now(21yo). I'm also using legacy-r and that's great board.
Also try to get a bigger board for you. You won't lose lots of performance, but you can use the board for 2-3-4 years. it won't be any problem. i consider you won't change a board per year, if it took 8 monthd to save for the board.
so my opinion in shorter: get legacy, as your foot will grow. get longer one, as you will grow up(weigth and tall).