The FJ Cruiser and the Xterra are both excellent vehicles; either would probably serve your needs, but they do have different strengths, which I think are worth considering before you decide.
Let me start with full disclosure - I have been a professional mechanic, fabricator, systems designer, and customizer for vehicles, and more specifically off-road vehicles, for almost 40 years. I have had the pleasure of taking apart, repairing, and upgrading virtually every 4WD vehicle available over the last 5 decades, so my experience has definitely influenced my biases and opinions of what are better or lessor vehicles. I currently own an FJ Cruiser, and have owned a long string of Landcruisers - none by accident. That said, here's what I think.
The Nissan Xterra is a fine 4WD sport utility. It was designed to compete with the more expensive Toyota 4Runner, providing a good, stable platform for less money. The Xterra, and most Nissans, have reasonably reliable service records, and are pretty well built. The Toyota FJ Cruiser, as well as most Toyotas, are IMHO among the finest constructed vehicles on the planet, regardless of price. Their reputation, and resale values, are well deserved. There is no accident the FJ Cruiser, despite being a low production vehicle, retains the highest resale value of any vehicle sold in this country. It is built like a brick, and offers off-road performance that has no rivals in it's price range, or pricing much higher, too. Every panel can be taken off and reinstalled without issue, and everything functions as it was designed, traditionally for ridiculously long periods of time.
The FJ does has limitations; some consider its visibility to be poor, fuel mileage is not great, and storage space is limited. The Xterra has more HP, better fuel economy (theoretically, at least), and more storage space - as well as 4 full doors for easier passenger entry and exit.
From an aftermarket perspective, the FJ wins hands down. Most manufacturers have ignored the Xterra, whether you are looking for protection, suspension, or aftermarket goodies of any type. This is due to demand; very few folks who have Xterras use them off-road - not unlike Range Rovers, G-Wagens, and many Landcruisers. The FJ was marketed directly to the off-road audience from day 1, and aftermarket demand and support shows that. While not as varied as the Jeep aftermarket support, options for most Toyota 4WDs are prevalent, allowing you an easier time to tailor the vehicle to your personal needs.
Bottom line to me - if you prefer a vehicle with more space, lower initial cost, and you are not inclined to do more serious trails or modifications, the Nissan Xterra would be an excellent choice. If you are more interested in a tank-tough off-road vehicle, with reasonable street manners, unmatched reliability, and lots of aftermarket support to outfit it for anything you might want to do, the FJ would be the winner, hands-down - and I did put my money where my mouth is
