My experience with tmobile and Att is probably reversed: tmobile found ways to add hidden or mysterious charges that others couldn't (I.e. $10 / month android charge). Most of the customer service reps in recent years got worse and worse.

I finally switched to Att about 2 years ago and I have to admit: every time I called, everyone was friendly and helpful. Nothing was hidden: they laid out their charges and I agreed to them. They may have even slightly higher ($10 a month), but I have excellent nationwide coverage and very good internet speeds when I need it.

This was just my experience, and it may be different for others.