First, are you familiar with
the guide (
unabridged and less dated) yet? It deals with cell phones plans and ISPs, among other things.
Second, it helps to know what you need in the way of average calling and texting when shopping for plans.
Third, FreedomPop is a datamining Sprint MVNO, and they use a more open-standards form of mVoIP for their voice services - similar setup to RingPlus (also a datamining Sprint MVNO), but without RingPlus' customer support. Republic is/was a datamining Sprint MVNO for their first three "generations" of service using a terrible, bastardized hybrid mVoIP implementation desperately trying to reinvent GSM UMA VoIP calling on CDMA with no proper technical support and locked down, proprietary handsets. Now, they're just another T-Mobile MVNO offering UMA calling on select handsets with extra datamining. It also means that although Republic now supports "BYOD", it's still limited to only support T-Mobile handsets that already supports T-Mo's UMA WiFi calling. All three options are likely going to require you to replace your current handset to use. Shop instead for a mobile plan that lets you keep your existing phone.
Personally, I don't recommend RingPlus, FreedomPop, or Republic - but if I had only those three to choose from, they'd be recommended in that exact order, from best to worst.
Instead, I'd be more likely to recommend US Mobile (T-Mobile), Ting (T-Mobile - if you need roaming and UMA WiFi calling), Ultra Mobile (T-Mobile), Airvoice (AT&T - unlock your phone first), H2O Wireless/EasyGo (AT&T - again, unlock your phone), or Puretalk USA (AT&T - same phone unlocking proviso).
Again, though... it's hard to know what to suggest without knowing what you actually need. And it's always best to pay for what you need.