Getting advice from others may or may not give you a good feel for exactly how AT&T would work for you. You can check coverage maps, but they are usually overly optimistic. If the map doesn't show good coverage, you can bet it won't be.
However, if the AT&T map shows good coverage, then I'd advise you to pick up a GoPhone refill card and SIM and get the one month $45 refill. That will give you a good idea of how the service will be, as the same towers and service is used. Also, you'll find out how it will work for you without having to make a commitment.
Sprint is a CDMA carrier, and the 950 and 950XL are not CDMA phones, which means you're most likely using VoLTE service or something like that to get service. It's definitely going to be worse than GSM towers, since the 950XL is a GSM phone with LTE. At least if you lose LTE, you can fall back to the GSM service, as long as there is coverage.
So, first, check the coverage map to see where you stand. Then grab a one month GoPhone sample. See how it works for you.