Hi Syracusa,
I would start with the mattress which is the most important of the two and also because different types of mattresses do best with different types of support systems.
There is more information about box springs, rigid foundations, platform beds, and other support systems that are most suitable for different types of mattresses in the foundation post here and the two posts that it links to in the second paragraph.
By buying the mattress first you will be in a better position to decide on which support system would be best to use underneath it.
Phoenix