Most of us do not have a device from each of the 4 major carriers. Most of us don't even friends with a device from each carrier. In your area it is more likely that two of the major 4 have useable service. Where I live Verizon is king and AT&T is a distant second followed by T-Mobile and Sprint is basically unusable. ZedoMax puts all 4 networks side by side in a speed test in the video above. These results are only good in San Fransisco and will vary everywhere else, but you can still get a good idea at the types of speed you should be able to get from devices in a major city.