If you're going offroading, I would keep the Toyota as long as it's a 4x4.
There are TONS of upgrade options for that vehicle. You could put a new suspension lift on it and have a very capable vehicle.
Most people who offroad their Xterras did so because they wanted a new truck that they could drive to work in, and take offroading.
Since you're 15, I would keep my expenses down by taking the truck and spend some money upgrading it. Why would you want new vehicle payments, and still be looking at upgrades?
Keep the truck, have money to get laid, and wait until you're 30 to buy something new.
Oh, and also, remember statistically you're going to get in 1-2 wrecks before you are 18 so keep something that's cheap to fix. (yes, I know that sounds like your dad).