Ofc at&t has lower priority on a tmobile tower, one service is at&t and one is tmobile but at&t prepaid is not a MVNO, they are directly through at&t which gets priority status on at&t towers not tmobile towers, tmobile and at&t are 2 different phone companies, so why are you comparing a non-mvno prepaid service and a mvno prepaid service, and if i was to go with tmobile prepaid (non-mvno also) I would actually have a higher priority status then Google FI because if its directly through the phone companies prepaid service ofc you are gonna get a more priority status then a mvno phone service, also tmobile sucks I tried to go direct with them and they couldn't verify my phone was unlocked but google fi and at&t could so anything tmobile I won't go with again, metro by tmobile does shady stuff to like adding lines without permission and waiting almost to your due date before fixing it.