Virgin Mobile is owned by Sprint and you are limited to Sprint's network.
Straight Talk is sold in Walmart and on Straight Talk's web site. In Wal-Mart, they offer phones that will activate on AT&T (separate HSDPA and LTE models) and Verizon CDMA. The AT&T activated phones will roam on other networks, the Verizon CDMA phones will not.
For $45/month, you can't beat it. High speed data up to the first 3Gb. Once you use 3Gb, you're throttled down to slower speeds until the next service period begins.
You should look at the terms of service first, especially the section on data usage.
Hope this helps in some way.