If you're going out west, be sure to check out the "Big 3"s coverage maps for both 3G and 4G. Some have very spotty coverage.
If good 4G is important to you, know that all 4G is NOT created equal. i.e. Sprint has two levels of 4G. LTE is the best and their older "4G" networks use something different that is much slower.
Virgin Mobile, Straight Talk and other "no contract" companies all piggy back off the big 3 towers. Within the same company their different plans will use different provider's towers. Millenicom uses both Sprint and Verizon towers, depending on which plan you choose. Straight Talk is the same. I believe I've read on forums that Straight Talk will even vary depending on what part of the country you buy their service.
If 4G is not important to you and you don't want to use your phone as a hotspot, check out Page Plus Cellular. They use Verizon towers and you can bring many (but not all) existing Verizon phones to their network. Their rates are very reasonable and they don't charge to reconnect (if you turn off the service for a while) like Millenicom does.
And be sure to ask about "throttling". Many of the no contract phones that advertise "free voice, text and data" will throttle your data speeds when you hit a certain point. When they do that, you internet access is almost useless.
Be sure to read the fine print on roaming. We traveled out west with folks that had a regular ATT service plan with 6GB data. What no one told them though is that ATT allows you to only use about 200 MB of data when you are roaming on another company's towers. Their smart phones got turned off with no warning because of this little known "gotcha" in the fine print. Fortunately we had Verizon and they do not do that.
The best places to get good info is the independent cellular forums. There's where you find out the "hidden fine print" Just spend some time with Google and you will learn a lot.
Hope this helps you with your due diligence.