@sertac if budget is not a problem then go for ones with full service history with agency- either FJ/X terra. If you are on a tighter budget then think of older Xterra ( 2010 and earlier you can get for <15k- depending on the condition, preferably the off road edition if you could get). I would opt for one without mods, no bumper cuts, and get it checked from a proper reliable garage before the deal and adjust the price according to the work needed to get it fit and fine.
Test drive both FJ and Xterra as they are similar same spec wise , except for the FJ Xtreme model. You might feel one car more comfortable to drive than the other, which can help you to decide.
FJ are more likely to be abused in desert than the Xterras in general.Good luck with your purchase!