T-Mobile vs Costco T-Mobile

Hi I have an iPhone 13 Pro Max on Verizon that’s paid for but I’m over paying so much and I’m looking to upgrade to the 17 pro. Would it be beneficial to upgrade via T-Mobile website/instore or go to the T-Mobile kiosk at Costco? Also how would get the best deal preferably getting the 17 Pro for free and keeping my military/nurse discount? It would be for 2 lines as I’m also interested in getting my gf an upgrade for her 13 pro max too.