Sounds like you are in a marginal signal area for LTE service, or service in general. Is it like this everywhere or just a certain place you are the majority of the time? I've also noticed that once a Verizon phone drops to 3G you almost have to put it in airplane mode to get it to reconnect to LTE, I think it's just a flaw in their switching algorithm or a hardware limitation. If you have weak LTE I would recommend purchasing a network extender for at home if you have home internet. It will at least give you a strong signal for calls and a 3 g signal for data, though you don't really need the data signal if you have wifi. You have to have home internet at least dsl speed and a usable gps signal/south facing window. The network switching all the time will kill the battery life of a device, it's def. worth looking into for a more reliable service.
All of that said, my iPad definitely holds a signal more than the iphone 6. It must have a weaker antenna or some issue with the baseband.