I'm actually waiting for a technician to get back with me about the proper travel length recommended for a lifted Xterra. As far as valving, Bilstein recommends a 170/60 valving for single shock setups.
You are running 5125s, those are 5150s.
IMO they are more for show, the performance over the 5125s is minimal. Its kind of like a 1/2 way step to a remote resi shock, if you want that, just go all the way.
Bilsteins with extra oil capacity are much better than the Bilsteins without, and for the price they look like a great idea. Problem is you won't know how the X handles until you try them. Give PRG a call and see if they'll test them for us. PRG is super honest and you'll get the straight skinny from them.
I know folks who run the 5150's in Jeep and they love them. I have learned though, when it comes to shocks, the cheaper the better. I am not baja'ing in the X.
Personally, I would not reply on a "salesman" to tell you on the correct length shock. I would get out there and measure it up for yourself. All you need is the drop and compress measurements which can be figured out with a jack and tape measure.
Replacement shocks are like shoes, some fit the application better than others, even if they are the same size, type, etc. Bilstein makes the 5150 series in a bunch of different lenghts and a couple different valving options. Be sure to get the lighter option for your X. They are NOT a bolt-on shock for your application either, you will need to order 3/4" id urethane bushings and cut 2 pieces of 3/4x.95" wall tubing 1.65" long for the lower mount. The upper stock Bilstein mount is fine.
A couple issues I've had with the 5150 series, the upper threaded mount ALWAYS comes loose and will let the shock swing side-to-side and it will rattle. Also, the paint (instead of plating) can chip and rust. You may have a major issue on the upper mount on the passenger side of your X, there is almost no room in there at all for the res. Here's my biggest complaint, the res on those shocks is indenticle to the internal res of the 5125 series, so moving it out side the body SHOULD allow for 1) more travel or 2) a shorter body, but Bilstein uses the same bodies and shafts as the 5125 series and they are the same overall lenght for the same travel. There is absolutly no advantage of running the 5150 over the regular 5125. They cost more, are harder to fit, rattle, and dont have as nice of finish.