While the S9/S9+ will work well in most areas they are a little less reliable in more rural areas here in the USA (at least on TMobile/its MVNOs as also verified here). I have driven 4 phones utilizing /e/ (S9, S9+, OnePlus 6T, Motorola G7+). For me (I am in more rural areas regularly) the OnePlus 6T and Motorola G7+ have been better because they support all TMobile bands. This advice might be different for AT&T/MVNOs as I haven’t tested this network.
HERE is some additional info.
One thing I dislike about the OP 6T and MOTO G7+ the ugly warning splash screens when booting due to the boatloader not being relocked after the flash. HERE IS HOW TO FIX . The S9/S9+ don’t have any if these ugly displays. My speculation leads me to think this is because of Samsung’s added Knox protections.
*Of course, if you want to use phones other than the S9/S9+ you will have to flash yourself. The majority of people will be good to go on the S phones from my experience.
*It would be great if anyone using AT&T/MVNOs could comment on that network. Edit: HERE you go