From what I've seen TerraGo doesn't offer support for external GPS units that is any better or worse than Esri's. All the GPS units listed as supported on TerraGo's site will work with ANY iOS app because the GPS unit itself conforms to the MFI protocol for Bluetooth GPS.
What TerraGo doesn't mention is if they offer any sort of deeper integration with high-accuracy units (like being able to access PDOP, RMS Error, Sat info). Collector doesn't support this at the moment either, but there are SDKs available from the manufacturers to add this to runtime apps, so I'm sure both TerraGo and Esri will be adding support at some point.