Justin, we use iPads and iPhones for field environmental work as well as other field applications. We have had good luck for several years with the Trimble R2 and R1 receivers paired with the Apple devices and we also have some Android phones that use the Trimble equipment as well. I am aware that Trimble has a new offering they call DA2/Catalyst where they completely rely on the Apple mobile device application software for all the position processing while using the low cost Catalyst antenna.
There are several GPS manufacturers and I am sure other brands work well. One thing Trimble does is that they have a specific ESRI/Trimble team dedicated to the Collector/Field Maps process that is helpful in resolving issues and faster implementation of enhancements. Your choice of a supplier with a good support record should also be a factor.
We have also done some disconnected offline data collection and have gotten it to work. Your danger there is data loss prior to synching back up to the server once back in cell signal range. Check the forums on those issues.
The R2 unit will do centimeter level accuracy under the right conditions and availability of an internet based correction signal. The R1 is a submeter pocket style receiver that also integrates with the iOS/Android devices without the need for a correction signal. The R2 will operate in the GNSS mode and collect elevations in NAVD88 if your setups are all correct.
Hope that is helpful. Good luck.
