Mike would like to have a cellphone that he can use over multiple carriers. Leo says that while everyone is moving to LTE and Voice Over LTE, we're not there yet. Half the phones are CDMA, the other are GSM. Verizon has a SIM card for modern LTE supported phones that will work over any network except Sprint. And the phones are unlocked by FCC mandate. So that's probably Mike's best bet.
If his phone doesn't have a SIM slot, then it's a CDMA only carrier. The iPhone 6 and the Samsung Galaxy Note 4 are likely the best phones for what Mike wants to do. Leo recommends starting with the carrier he wants, then choose a phone.