Mobile Share Value Plan? And you'd be taking the contract from someone else? Hopefully, they're not paying extra for it. If you're not paying the extra on their contract, then obviously the $ decision is to take the contract, because you'd be getting the phone cheaper than you could anywhere else. However, that phone is not going to get unlocked until the end of the contract (unless you pay the ETF to terminate the contract), so not being able to take it to Tmobile anytime in the next two years is something to consider if that is important to you.
AT&T Next on a plan like yours is iffy. Next is basically a 0% financing contract with an option to trade the phone in early instead of paying the remainder of the payments. So a Next 12 contract has 20 monthly payments, but once you have made 12 payments, you can trade the phone in instead of making the remaining 8 payments, as long as you are starting a new Next contract. With a single line that is currently under contract, you would have to pay the ETF to start Next on a new phone. You would also only receive $15 discount for not having the phone under contract, saving $360 over the life of a 2 year contract, but the subsidy you get from a 2 year contract is $450.
You could also do the Apple Upgrade Program, which is also a 0% financing contract with an option to trade the phone early instead of paying the remainder of the payments. It is a 24 month contract, with the option to trade in after 12 months as long as you reup a new contract. This would get you an unlocked phone that you could take to any carrier, if that appeals to you. Depending on the phone you select, it may actually be cheaper on a per month basis than Next 12 even though the AUP includes AppleCare+ (it's cheaper because it is 24 months instead of 20, but if you trade in every year, the remaining months on the contract don't matter).
I'm afraid I can't tell you much about the Tmobile options, because I'm not at all familiar with Tmobile. We went through all these calculations last year around what was cheapest with our accounts on AT&T, whether we should do Next instead of contracts, etc. so I know AT&T pretty well, Tmobile not so much.