Ok, coilovers are brand new coils spings. A spacer lift uses the stock coils by preloading the the springs with usually a polyurethane spacer. You put the spacer on top of the coil. You can go up to 3" on a spacer. Coil overs give a great ride, while spacers stiffen the ride a little. A lot of people like having a stiffer ride...its just ur preference.
A lot of people have the Revtech spacers and like them. Plus, spacers are A LOT more cheaper than coilovers. If u have the money and are looking for comfort...go with coil overs. If u are looking for a cheaper way, go with spacers. Spacers can eventually cause the springs to sag, but this is if u do a lot of hardcore offroading with them.
Check out this sticky post for more info