How is the latency? Low latency will make it possible to do everything over the data connection, and get rid of voice altogether. Not that these 4 will ever offer data-only service on a phone; but if it's offered for tablets, someone can make a Skype phone that uses LTE and Wifi but no voice.
·Verizon Online DSL
From what I've seen so far, the network technology (HSPA+ vs. LTE) doesn't impact latency as much as the network backbone choices of the provider. Networks with more regionalized backbones (e.g. Verizon and T-Mobile dumping Denver traffic to the Internet in Denver, vs. AT&T and Sprint routing said data far and wide) tend to have better latency than those who don't...VZ LTE in Denver can now hit 70ms reliably and 50ms every once in awhile. T-Mobile at times is even lower...I've seen 22ms to local sites with HSPA+.
How does this translate experience-wise? Well, I've used Skype over LTE on multiple occasions and it has been perfectly solid. I've used LTE and HSPA+ for online gaming (League of Legends...it's a latency-sensitive RTS/RPG) and they've worked well enough. Then again, I've done Skype calls (and used other VoIP) over 3G plenty of times, many of them successful.
tl;dr: T-Mobile and Verizon latency are excellent. Sprint is less so at this point. AT&TT isn't from what I've seen, though I haven't been in one of their LTE areas with an LTE device yet.
reply to axus
I can currently make very reliable and clear google voice calls over Verizon LTE with groove ip, unfortunately verizon doesn't offer a smartphone data only plan or I could drop the verizon minutes and texts.
This is one of the reasons data is so expensive on LTE, and AT&T and Verizon don't allow data only sim's to be used on smartphones, they don't want to lose the voice/text revenue until they have us paying a lot more for data.