Costco or full service T-Mobile store?

Wife and I along with two grown kids have used a family plan for many years with Verizon. I keep reading that Verizon is dropping some discounts and raising rates for many. My local Costco has AT&T in the store and AT&T sucks where we live. (Had a work mobile with AT&T) Many friends are telling me T-Mobile works great in our area and pretty much everywhere they travel. I'm ready to switch. Does it make sense to use a Costco dealing with T-Mobile or go into a T-Mobile full service store? I owe a little on my phone, wife has a new I-Phone and unsure about my adult kids. Just wondering which is the better route? Costco or T-Mobile store?