The preferred network type depends on the type of coverage available at your area. For example, if there's 3G/4G LTE service available, you should set your network mode to "LTE/CDMA".
But let's suppose that you only have 3G coverage and you have a 4G LTE-capable device... Then placing your phone on "CDMA only" would be your best choice to have a better network connectivity experience as leaving the phone on the default network mode (LTE/CDMA) will cause some intermittent connection issues.
I hope this information has clarified your doubts and if there's anything else you need to know about network-related topics, please let me know. I'm here to help!