From what I've seen, they are both reliable, and do well off road about the same.
The main complaints from those with them has been visibility...lots of blind spots, and off road, hard to maneuver with the hood blocking the view up front, and the high belt line windows blocking the view out the back and sides.
The X is faster on the road, with better handling. The FJ rides softer though.
The FJ has better protection of electrical like the alternator, and the intake for the engine is better protected, but the diffs, etc, are about about the same in protection.
The differences are such that you could probably get which ever one you prefered, and be happy with it....depending upon how important the above factors are to you.
The main complaint about the FJ from people who didn't buy one, is that everyone thinks they're gay....and even straight people who like the FJ shy away...kind of like straight people who are afraid to wear a pink shirt to work...even if they think they look good in pink, etc.
So - if you like the FJ enough to put up with that, hey, as a truck, its OK.
If you find later that the stylish exterior is not as much fun as you'd hoped...there's no problem reselling the FJ's, as the soccer moms ADORE them, because they're SO cute.
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- TJ
2001 Xterra '03 VG33, SE 5 spd, 305/70/16's, Revolvers, UBSkidderz, Doubled AAL's, 3"SL/2"BL, winch/bumpers, skids, sliders, OBA, Snorkel, pine stripes....
Friends don't let friends drive stock.
http://www.gifsoup.com/view/501230/tj-tackling-crawlers-ridge-o.gif