Well its

A.) Sale SI, get shitty car, so I dont have to make payments save up buy Z

B.) Keep SI make payments, and save what I can from ti me to time.

keep your SI, but when you are ready to get the 370Z, try to sell the car yourself as the dealer will only give you 11-13K for your car.

Do you think its worth it?

I would sale car, get shitty car save for 1 year