I'm not sure why Sprint is even in the conversation -- it was sold to T-MO and no longer exists.
The CDMA technology it (and Verizon) employed is close to being completely eliminated by the end of 2022 in the U.S. Verizon started its sunsetting process for CDMA phones a few years ago by ceasing to activate new phones that relied on it.
The service is gone, and soon, the technology will be as well (though apparently, DISH and its lawyers didn't get that memo, but that's another topic altogether).
An unlocked global LTE/GSM phone can be used with any of the three major U.S. carrier networks, or the prepaid/MVNO services that piggyback onto them.
The choice between the AT&T, T-MO, or Verizon networks is a matter of service quality in the specific use area, and the choice between service options, prepaid or postpaid, comes down to desired features, airtime, and cost.
The market shift away from service contracts has rendered postpaid plans more flexible, but they are still more costly, albeit with greater features.
For a temporary resident, choosing the prepaid/MVNO route offers the greatest flexibility, at lower cost, albeit by sacrificing some features, and service quality (lower speeds and priority).
Verizon is more friendly to MVNOs than it used to be, but AT&T and T-MO are still better in that respect.
Between the latter two, T-MO is at a disadvantage because it has been historically lacking in low-band coverage, the type needed to penetrate buildings, which affected its coverage quality. It has steadily addressed that by purchasing more spectrum to improve service, but part of that effort includes the 600MHz Band 71, which the global A2399 model does not support. That could be a potential hindrance if the phone is used in an area where Band 71 is deployed to provide coverage.
The beauty of unlocked phones and prepaid service is the flexibility that they offer. But in terms of getting basic service, there should be no hurdles, just as it is when U.S. travelers go to Europe, and buy temporary SIMs there during their stays.