CA is a mighty big place, and I haven't traveled all that much of it. However I do happen to have phones on ATT's 3G network that can act as hotspots, and USB networking devices for VZW and Sprint. I don't have T-Moble because the coverage map looked like it wasn't really useful. I also have an RV, and have left "major city areas" quite a bit. I don't currently have any 4G networking gear (unless you count all the 3G stuff the ITU reclassified, in which case I have 4G but no LTE).
In my experience there is a lot of coastline that has no service from anyone. There are some inland areas where hills or mountains block signal from everyone. Some places I can get ATT but not VZW. Some places I can get VZW and not ATT. Around the coast it was pretty even. In land VZW seems to have a little bit of an edge, but not a lot. Many places I could get VZW and/or ATT but not Sprint. I can't recall any places I could get Sprint but neither ATT or VZW, but looking at coverage maps there might be such places, I've just never attempted to get signal there. However there is no substitute for actually testing in your location. Everyone has said VZW has the best coverage for years, but for years my house had no VZW service, while it did have spotty ATT service (recently VZW started serving the area, and also around the same time ATT's service picked up a lot as well)
Most places where I could get ATT and VZW 3G the ATT was faster. Sometimes it was even faster if the device showed "fewer bars".
The "reasonable best option" I would see is to get one device on ATT and one on VZW, and ignore the rest. My VZW device came form http://www.millenicom.com/ I don't know if they still sell them or not. They use to have a $50/month plan for 10G. It looked like a no-contract plan, but the way it was set up when you stop paying the monthly fee they want the device back or hit you with a big disconnect fee (and they charge for the device up front), however even so it was still a bit cheaper then other VZW data devices, just not by as much as it first looked. Things may have changed since then, so look around, but make sure you give them a peek. My ATT device is an iPhone (was a 3GS, then a 4, now a 4S...my wife and I take turns getting a new one each year). Another option is the new iPad, they have large up front costs, but a month by month plan (no fee to cancel, no fee to restart). From what I have read on the net only the VZW one currently supports hotspot sharing, ATT still hasn't gotten their ducks in a row there. Depending on what you want to do with the internet you might be just fine only having access on the iPad anyway though.
I have no data for Nevada. Last time I was in Arizona I didn't have a VZW device, but ATT seemed fine pretty much everywhere.