The longer you make payments, the more money you are paying for the truck in interest. Payments on my '02 F-150 with 25k miles were $10 less per month than the '98 F-150 with 70k that was sitting beside it, but they last a year and half longer.
Personally, I would keep the FX4. The only thing you would gain with the other one is a warrentee... that you will probably never need. According the the Oasis report, the only thing my '02 had done under warrentee was it had a horn replaced and a light bulb in the map light replaced. The only thing that has broken since I have had it (4 years and 50k miles later) is the light switch knob that broke off in the dead of winter, and it was $20 out of my pocket to replace, two license plate bulbs and the third brake light bulb, all three was less than $1 to replace.
I still get crap in the mail trying to get me to buy an aftermarket extended warrentee, so I would imagine someone would want to sell you one too. Be forewarned, any modifications can cause them to void it on you, factory or aftermarket.
Take care of it, don't abuse it, and keep up on the maintenance and it should be a very reliable little truck for you.