Does the stipend from your university have any teaching or research assistantship requirements? If so, I think your big decision is regarding if you want to work your first year while taking courses or not. Some people prefer to focus just on their classes in the beginning and so they use their NSF at the beginning while they adjust to grad school, while others like to save their NSF for when they are doing fieldwork or writing up their dissertation so they have no teaching or work requirements during that period.
Another thing to consider is how many years of funding your university provides you and if they are strict on which years they provide you with funding. For example, if they provide you with funding your first 3 years and you are responsible for applying for grants your last two years, but that is a question for your university.