Cdma auto prl will switch you between 1x and 3G when; from what I believe the signal for 1x is faint. Evdo only will only look for towers that support 3G other than that, no signal. Cdma only is 1x pure. The thing is if you have less than 3 bars from what I read, your battery drains faster. So unless you are digging for a signal, stay on cdma only, unless someone can correct me.