You are missing a couple of things. 40 upgrade fee. That's 200$. Difference is only 800 for 2 years. But you are comparing Apple and oranges. You have a lean plan. 450 min. Rollover is only good for 12 month rolling and you don't use text.
Your going into an unlimited plan. With 10gb that u can tether. It's completly different if u want to save money stay where you are. But I would think with the new phones people would use more data.
True, the upgrade fees do factor into it, and I get that the family share is in some ways more versatile, but we've been okay with this plan so far. It's not worth $800 to me to be able to tether (which is essentially the only feature I would like). I have one of the T-mobile free 200mb sims in my iPad mini that I can use for tethering when necessary.