A spacer kit gives you these polyurethane "pucks", or big round pieces of poly that you put
between your springs (rear) and struts (front) and the surfaces where those items normally mount. This in turn raises the height of the vehicle by spacing the distance between the vehicle itself and the axle components.
The full suspension kits replace the actual springs (and various other components) with taller and usually better riding springs that are a bit longer. Their extended length in itself raises the height of the vehicle.
The full suspension kits cost more, but usually are a more complete and better engineered way to lift a vehicle if you plan on running hard off road (or off road at all).
Some spacer lifts change a Liberty's front axle/suspension angles in such a way that they cannot operate as designed, which can lead to premature wear and failure of various components. Other spacer kits are designed differently and will work for most people who drive 90% street, 10% mild off road. Daystar seems to be a favorite poly spacer kit, along with something called the "combo" kit from Rocky Road Outfitters. Here are links for you to look over and consider...
http://www.rockymountainsusp.com/jeep_Liberty.htm
http://www.rocky-road.com/kjsus.html
http://www.boulderbars.com/frankenlift.htm
I just bought my Liberty two weeks ago and have been searching this forum and many others seeking past experience and advice on all the different lifts out there. Personally, I am planning the "combo" lift from Rocky Road because I don't plan to use my KJ off road much and it's cheap (and I'm cheap).
Good luck with your decision.