I'm currently using a Digital Antenna dual band cell repeater that I just installed in my van before the trip. Works great with only one dead spot from FL to OR and that was in a very remote section of WY.
The major difference between a repeater and an amplifier is this: a repeater uses an inside and outside antenna so you are totally wireless as the inside antenna picks up the signal from your cell phone, transfers it by wire to the amplifier and then again by wire to the external antenna.
An amplifier system does the same thing, but does not have the internal antenna, requiring you to hard wire the phone to the amplifier. They are said to work better due to having everything hard wired together with less signal loss, but I found that the repeater system works fine for my needs.