In general lower frequencies penetrate buildings better than higher frequencies - for example, table 1 in this link:
http://www.vilicom.com/wp-content/uploads/2012/10/Penetration-Loss.pdf which shows a 12dB loss at 2400MHz, an 11dB loss at 2100MHz and a 9dB loss at 900MHz. There are other similar papers out there showing the same thing. That said, there are certain frequencies that correspond to the absorption wavelength of certain building materials that and so "lower is always better" isn't always true... but it's generally true.
Second, being closer to a higher frequency tower will result in a better connection than being farther away from a lower frequency tower. So where the towers are relative to where you are is more important than the frequency... which is why people in major cities can have great reception at >2GHz inside of buildings.
Back to the original question, swanysto, if you want the best coverage you want the network with the best tower coverage for the area that you are at. In general across the US, that's Verizon, but specifically where you are it can be AT&T, Sprint or T-Mobile. In general, you will have the best service with Verizon, but there will be times when, for example, the AT&T tower is closer and the AT&T guys have better coverage... and if that specific coverage is more important than broader general coverage, then you could consider switching to AT&T.
Towers don't all cover all frequencies - in fact, they usually don't - so if you want to totally geek out, you can pull up the maps (which can be hard to find) of which towers are on which network (2G, 3G, LTE, etc) and which frequencies and optimize your phone for that.
WiFi calling helps a lot - but only if you have WiFi and only if your phone and network support it and only if there is WiFi where you are at.
GoogleFi picks the best tower or WiFi between Sprint and T-Mobile and uses both networks and will use WiFi if that's available so that can be a good option.
To maximize your odds of the best coverage, always choose the phone with the most supported frequencies and the one that has the most networks (2G, 3G, LTE) coverage. So in general when you are picking phones and you need to decide between two phones, look at what bands each covers and pick the phone with the broadest coverage.