http://www.jeepforum.com/forum/f27/what-difference-between-mini-spool-spool-detroit-locker-331474/
Hopefully this helps, but the long story short is:
1. Spools lock the left and right wheel together 100% of the time
2. Spools are stronger than lockers, due to less moving parts (spools are stronger than mini-spools)
3. Spools are lighter than lockers.
Lockers are beneficial because;
1. On-demand traction, but the option for the differential to work as designed, while on the road. (THIS IS VERY IMPORTANT FOR THE LONGEVITY OF YOUR TIRES)
2. In an off-road situation, you do not want to run locked 100% of the time, because your turn radius is affected (effected?), a tight turning radius is an important function of and open-differential.
3. If you are running alone, and locked, the difficulty of recovery will be greater. By this I mean, you will get stuck more frequently in an unlocked vehicle over a locked vehicle, but you won't be as stuck, making recovery with a winch a lot easier.
I'm sure most will agree with me when I say; I would highly recommend not putting a spool in your Xterra, you will ruin the drive on road, and most of the time extreme traction is not necessary. Lockers are more expensive, but well worth it. Feel free to disagree, I've never heard of someone running a spool/minispool in an overland vehicle, so I may be missing some positives to them.
Hope this helps!