I am not sure what the trade off on T-Mobile is assuming you live in an area with good coverage. For $50 a month you get unlimited data, texting and voice. The LTE is capped at 500 Mb's and then you drop to 3G or Edge, but you have free tethering, international calling and data, and HD voice. For $10 a month more your unlimited data includes 2.5 GB's of LTE. Additional lines with all the same features are $10 to $20 a month more.
If you don't do much voice T-Mobile can do even better than that. 100 min/month voice, unlimited messages and data (5GB LTE cap) is $30/month. Even with no discount for additional lines it's still a great deal at two lines, and possibly even at 3.