One other thing to consider is recurring costs, such as insurance. Is the 350Z going to cost you more for insurance as a teenage male than the G35? Do you plan to do the maintenance and perform most repairs yourself? If not, do you plan to use an independent shop or the dealership? If the dealership, then are both dealerships local to you or only the Nissan dealership? Since both cars you're currently considering are older used cars I don't guess there is warranty involved, but even though Nissan is the parent company of Infiniti, covered warranty and recall work cannot be performed on an Infiniti by a Nissan dealership so that may be something to keep in mind as well.
At some point, if you have friends and enjoy driving, you'll wish you had more than a 2-seater car for taking road trips with friends.
-Rod