Switching to ST is a great idea. Get the Tmobile SIM. Not because of the "cap" or whatever but because T mobile offers MUCH better data service than Att. But you have to make sure that Tmobile has coverage in your area. I did a 2 month test when I signed up for ST the first time. I used the ATT sim the first time and the Tmob sim the 2nd month. The ATT sim was giving me 3mbps and really good coverage. The Tmobile SIM is giving me 8-12mbps and decent coverage. Att does have the coverage, Ill give them that, but their data speeds are awful. T mobile is MUCH better with data speeds.
I don't know how much you talk on the phone but Tmobile offers the $30 5Gb plan which a lot of people use including myself, although I am switching back to ST at the end of the month. 100 minutes is enough but It's always on my mind every time I make a call.
Your best bet is to Google and see if you can find other people using an Oppo phone on T mobile or Att and see what they are saying.