This is your decision to do so. If you want the latest UI with the newer maps, then Sync 3.4 is it. If you want a system that is OEM to the vehicle and has the latest Sync and map updates, then stay with Sync 3.0.
Either way will work fine. It comes down to personal preference really.
If you update those to Sync 3.4, Ford could refuse any warranty claim on the APIM (APIM ONLY). The rest of the warranty would still be fine. Just keep that in mind. However, replacement APIM’s are only about $300-400 for newer NAV units, so weigh your options there.