Sewing question for the Interwebs

[amethyst73]

I have a shirt pattern that I want to make for myself, and I have a question about sizing.

My bust measurement is (I think) at the top size of one size set (8-10-12).  My waist measurement is at the bottom of the next size set (14-16-18).  Which measurement should I pay more attention to when I buy the pattern?

(I went and bought the 14-16-18 size yesterday, but haven't done anything besides look at the instructions, and I want to know whether I should exchange it for the next size down before getting started.)