Best thing to do is get insurance quotes for each car and go from there. I don't understand how an STi is less to insure than a regular WRX but whatever. The trick with being on your parents insurance is to name one of your parents as the primary operator which is perfectly legal.. (that is what i did with my eclipse) If you do that then the premuim won't be nearly as high as you think. See if it's cool with your parents if you do it that way. Whatever the increase in insurance from your mustang just float that $$ to your parents to make up the difference.
I'm 27 and i pay $140 a month for full coverage on an 05 STi, 08 WRX, and renters insurance. which comes out to about $75 a month for insurance on the STi, $56 for the WRX... which will become even less in the next couple years (i'm getting old)
Just something to think about.