Suggest you go with what most on this thread are telling you. You'll be taking a huge depreciation hit on your trade or private sale of your car.

Keep your sedan and mod it. You can get beautiful rims and tires, put some coilovers to lower it and you can get sway bars which will improve your handling performance. In addition you can put the steering wheel shifters on the car as well. Just do a search on "steering wheel shift paddles" on this site and you'll come up with quite a few hits.
If your looking for some more getup and go bullydog makes a tuner for our cars which is getting some very good reviews on this site.

In the end you'll have a car that suits your needs, personality and desire for performance.

 

yeah i can (somewhat) understand owners of older gen g35 trading in to get the current g37 but i will never comprehend your situation. you can easily put that mod into your sedan and make it significantly handle better and/or look sportier then the coupe.

 

Rather than trade it in -- sell that sedan private and someone can get a basically brand new sedan for thousands less than a highly negotiated price at the stealership.

I'd list it in the papers/cars.com in your area.

what ever happened to the "rainy day" fund? Guess the car payment trumps that....