I am another both. No data plan on the iPad and an UNLIMITED data plan on the iPhone. iPad is used for everything that maps / charts would be in and out of the airplane. iPhone is used for BACKUP in the airplane and latest weather on the ground where there is no WiFi.
The FAA does not mind if we are Paperless but they recommend a backup. The iPhone is the backup as it has the same maps / charts in the air. It is just harder to read on the small screen. IMHO, the Backup is the main reason for the iPhone. I really do not want two iPads to get the redundancy without paper.
Only paper in the airplane are LOCAL maps, charts , VOR test log, GPS instructions, SL30 instructions, Check List, and ARROW.
ARROW = Airworthiness, Registration, Radio, Operating Limitations, Weight and Balance.
iPad also has PDF manuals for all radios and many of the airplane systems.