D.W.
Given your daughter's age I think you should talk to her. If you can only afford one party then ask her what she would prefer.
If I were turning 18, I'd rather have a birthday party more with my friends and less on the grandparents/aunts/uncles scale and have something on a smaller scale-like dinner with family or a bbq for my graduation.
Ask her what she would like. That's where I'd start.