Which carrier is better? Sprint or T-Mobile?

I live in the LA County in California, and the internet speeds I get often slow down. Sometimes I don't even receive 3G data on my iPhone 4S. I was thinking about switching to T-Mobile, because I hear some people saying the coverage is better and the value is good. Any thoughts?

Verizon is pricey, but fast. My phone can reach up to 25 Mbps on LTE

T-mobile is good, good coverage, good customer service, 4G, 4G LTE

Local coverage is probably better but T Mobile sucks for long distance travel.

Sprint. I don't have it but my sister HAD tmobile and they would charge her so much for Internet usage even though she paid for it.