I just went through this decision process and opted to switch from T-Mobile to ATT.
I created a spreadsheet for myself - This was critical in forming the final plan.
I originally was going to stick with T-Mobile as I prefer a 2 year deal vs 3, but
once I plugged in the numbers with all the discounts, i.e. monthly device credit, monthly switching credit and $100 per line Costco gift card, Att was overall cheaper over 3 years. This included some scenarios where I looked at paying off devices after 2 years and doing a year of say Cricket (or Mint in the T-mobile scenario). In my situation, I was trading in 3 devices (iPhone 12 and 13's.) So the device credits were a big factor. Switching from another Carrier to ATT attracted a $275 per line credit as well. All ATT credits pay off over 36 months. So you really need to look at the monthly cost, and the 2 and 3 year cost scenarios so you are comparing apples to apples.
So nutshell:
Moving to ATT from T-mobile and upgrading 3 phones to iPhone 16 Pro level resulted in a monthly payment that is about $150-$160 a month. (and still have not factored in $300 of Costco gift cards.) This was for the Top Cell Plan - price reduced $10 a month with ATT Signature Plan bringing it to the same price as the Middle Plan. (AARP Discount in this case.)
As to the phones, before someone asks, the phones we upgraded TO are: 2 iPhone 16 Pro's(256gb) and 1 iPhone 16 Pro Max 512gb. This overall Pricing scenario really worked for me. YMMV.
Incidentally, I found ATT cell service actually working better in my area for the places I tend to frequent. So that is a win.
I don't think getting a deal and hating the cell service is a very good idea. Before I considered switching, I tried the Cricket Free-Trial and confirmed that I usually saw the same if not more bars than T-mobile. For those considering T-Mobile, they offer a Trial run as well. I am not discussing Verizon as they had annoying dead zones for us, so while they let you trade in really crap phones sometimes, not worth it to me to not have service when needed. Also a 3 year deal.
Full disclosure: I bought my wife her new phone free and clear direct from apple using Apple Card and their Monthly installment plan. Reasons. 1. Didn't have another phone to trade in, as hers was too old. 2. She wants to be on Verizon network when she travels. So she now has an unlocked phone and is on Visible for $25 per month. (downgraded from Visible+... she hasn't complained yet!!). Generally I have found Visible pretty good, and Customer Service was good when there was a glitch with the plan downgrade.
Final Disclosure: I found the ATT retail partner at Costco to be very good. Nothing shady happened. Others have reported issues at their Costco's. I went in VERY PREPARED regarding what I wanted and did not want. At my Costco, a few different sales folks rotate through the ATT kiosk. I just happened to meet them all at various times and while one of them gave off a shady vibe, the one I worked with was very eager and honest. She really knew her way around her app, my t-mobile app, and did the whole deal really fast and professionally. I hope if you go this route, you have the same great experience. (My sales person even invited me to take pictures of her screens as evidence of what I signed up for, what discounts I was expecting etc etc.). Also, after all the form filling out is done, they could not execute the deal until you get on the line with a VERIFIER and they run you through everything you are signing up for and expecting. So there are a couple of moments (at least in my case) where you can nix things like the upgrade program, or their protection program, if a shady sales person tried to add them on. Again, YMMV.
Good Luck!