First, the phone is far from useless with Republic. It can currently be activated on the GSM partner and soon will be able to be activated on Sprint as well.
Second, Fi may be a fine choice if one needs the things Fi offers (international roaming, the perhaps enhanced coverage of multiple networks, etc). If not, in almost all scenarios, Fi is more expensive than Republic. In addition, if one relies on wifi for coverage in any meaningful way, the “wifi last” configuration of Fi doesn’t work so well as it will stubbornly prefer even the weakest of cell signals in making a call.