Not too many people get pre-paid smart phones. The plan choices are pretty thin on the GSM side. You've basically got T-Mobile or Straight Talk, depending on the network coverage in your area.
If you have infinite money and need 50Mbps wireless access anywhere in the US, then of course you get Verizon. But if you just want a phone that works in major cities, then T-Mobile can't be beat. (I've used both, and I did love my unlimited data on Verizon through my employer. But T-mobile is half the cost and just as good for how I use my phone.)
Also, does Straight Talk offer LTE as well? I think AIO comes with LTE. I also liked AIO's $70 plan as Straight Talk is limited to 2.5GB of 4G data on the unlimited plan.
(I use Straight Talk but was thinking about getting Nexus 5 which comes with LTE)
I'm planning to switch to ST when my AT&T contract expires.