Your probably aware that you will take a bath on the trade in, somewhere between 4K and 8K on the loss. IMO, I would keep your ride and enjoy it the way it is. BTW, do you have the OEM 20's wheels and tires?
If you plan towing anything in excess of 2500 lbs on longer trips, just forget it. Your transmission would become hot (not warm, but HOT) potentionally resulting in early transmission defects. I gave up towing my travel trailer (3300 pound as weighed) after the second camping trip (less than 50 kms) and let my Ford F-150 Lariat do this job... Not as "sexy" as my CX-9, but even the fuel economy numbers will show you that something is just not right when towing this weight with the CX-9:
Conditions: Same speed (90-100 km/h), similar terrain (hwy, country roads, rolling hills), same load (3300 lbs).
Mazda CX-9 (3.5L engine): 22.8 L/100 km (10.3 MPG)
Ford F-150 Lariat (5.4L Triton engine): 17.8 L/100 km (13.2 MPG)
And again, my CX-9 is equipped with the tow package...
I agree with all the prior posts. It doesn't matter what you owe, or even if you currently owe nothing, you will be taking that hit in depreciation some way or another; the car is worth that much less after a year.