If you're in school, I say keep the car until you graduate, and not get into a monthly payment. The 325i is plenty reliable, especially if yours has been good so far. I doubt the BRZ is even quicker.
if money is tight, I don't see how having a car payment will help your situation. when I was in college, I almost bought a used sports car.. I figured I could easily make the payments with my part-time job.. In the end I didn't, and I'm SO glad I kept my old car, because I had enough cash from a summer job that let me quit my part time job at school my senior year and I just had a lot of fun instead, and didn't have to worry about a car payment.
Also, whoever said insurance will be cheaper on the BRZ, I bet that's not true. I had a 13 year old integra coupe when I got my E90, and the insurance on the E90 was LESS than the old integra. New sports coupes driven by lots of 18-25 year old males tend to cost a lot more to insure.