Hey Falls, I will try to explain it the best I can. I'm sure there are others that can explain it better.
With a spacer lift, you are basically just pre-loading the spring so at normal compression, the suspension is extended and the vehicle is lifted.
The problem with spacer lifts, or if if you just add new springs for the lift but not shocks, is that most stock shocks can not extend as far as the springs can travel. So if you are off-roading and the springs are fully extended, the shock will bottom out because the spring can travel farther than the shock can.
Spacers are a good fit for people who want to get a lifted appearance, want larger tires, and don't want to break the bank to get a lift. Spacers are also good for people who don't necessarily off-road all that much. The good thing about a true suspension lift (springs and shocks) is that you get the benefit of extended travel along with the lift.
I'm a big advocate for the OME suspension. For under $800, you can get a true suspension lift that not only yields one of the more popular lifts on a 4Runner (3" up front, 2" in the rear) but also one that is practical if you off-road even a little. It rides a little stiff but considering it keeps the 4Runner flat in the turns as good if not better than a 4Runner at stock height with XREAS, it is a trade-off I'm more than happy to live with.
Coilovers are great but I think they are overkill (and pricey) for most people. Even the most competitively priced CO setup is almost twice the price as the OME setup.
Based on the fact that you have already put some money into your current lift and have given thought to a bigger lift, I would suggest going with the OME lift. I know you have a similar looking 4Runner as mine so have a look at my build thread with pics of my 4Runner going from stock to a 1" leveling kit and 32" tires to the OME lift with 33" tires.
http://www.toyota-4runner.org/gallery/64741-g-i-g-s-build-thread.html
Hope that helps
