It doesn't matter, anyway. Sprint and T-Mobile operate essentially incompatible networks. Your Sprint phone is, with rare exception for "road warrior" phones, physically incapable of operating with T-Mobile's equipment. I don't think any Sprint iPhones fall under said exception.
You can, to some degree, use Sprint handsets on Verizon (and vice versa, if Sprint were to allow it, which they do not). I've heard of some success getting Sprint to unlock some handsets for that purpose by complaining hard enough and eventually getting somebody who even knows what's involved. Verizon will blindly "activate" anything remotely compatible if you give them the IMEI, but they can't guarantee that it'll work nor will they support it of course. Sprint and Verizon also use different LTE bands, IIRC, so you probably won't be able to use a Sprint LTE handset on Verizon in any case. It's generally easier to just sell the used handset to another Sprint user who can then upgrade off-cycle and under contract then start anew with your new preferred provider if you're coming from or going to Sprint or even Verizon, for that matter.
AFAIK, the few multi-standard ("road warrior") handsets that Sprint sells do have unlocked or at least unlockable (upon request to Sprint) GSM/UMTS and maybe LTE radios. Any SIM should work once they've been used once on Sprint. The CDMA2000 radio is still locked to Sprint, though, and the WiMax radio, on models so equipped, is also usually locked to Sprint/Clear, if that's even relevant anymore.