As stated, the gas mileage is within a few percent, same grade fuel unless you do 91 octane tuning. And the power difference is tremendous. Overall the GT is a better car when it comes to handling, braking, accelerating, etc. And handling and braking are safety concerns too.
Insurance SHOULD be minimally different. The cost of repairs are similar between the models.
Buy what you can afford. If you need to have a warranty and are looking used, then just go with an aftermarket warranty. Shop around for prices on that too, and check with your local dealer to make sure that they accept that warranty company.
GT's can be had used and unbeaten for under $18,000. If it came down to price I would take a used GT over a brand new V6. I wouldn't even think twice or regret it. You are getting all the options for your money. And when people ask about your car you don't have to say "I wish I had bought the GT but mines just the V6."