New car v used car I need guidance

A

Going to get a new car soon I want a jeep grand cherokee and have been looking at new and used vehicles this is a car I do plan on keeping for a long while I'm just wondering would you rather get somthing brand spanking nee or a used car with less than 30,000 miles? Also if you were to get a used car what's the mileage you wouldnt go over?