I'm with TMobile right now, but I've been with Verizon, Google Fi, and Sprint(pre-T-Mobile).
Porting numbers is easy. So many are now using the same networks, so coverage hasn't been an issue for me in 10+ years.
You basically need to run the numbers. For example, I currently have an older plan from T Mobile which is unlimited everything w/3 lines for $120/month. The plan also includes basic Netflix and Apple TV for free. So I'd say I get about $110 in value, which is $36/line. They won't give me any deals on phones unless I switch to a higher priced plan. When the time comes for a new phone, which will cost $1k each, I will shop and see what kind of deals I can get, or I'll just buy the phones outright, and switch to a cheaper carrier like Mint. It really comes down to the math for me. If someone will offer me a $1k phone for $300 (x3), that's $2,100 over 36 months. So roughly $60/month. If I can get Mint for $90 for 3 phones, that may be a better option for me.