Google Nexus 5X
Matthew is looking at the Google Nexus 5X but he's also looking at the OnePlus X. Leo says that both are good, but the Nexus 5X has the benefit of having a fingerprint reader. If he can do without that, then sure. Leo also recommends the Motorola Moto G. The One Plus Three is about to come out too, so if he can wait, it might be worth it.
Mark got a new Google Nexus 5X and Sprint says they don't have that phone on their network. Leo says to just ignore that. All services have converted over to LTE and as such, they all have SIM cards. So if he gets a SIM card from them, it should work.