I found the big iPads very uncomfortable to use in small cockpit and installed SD on the iPhone 5S. Combination with the desk version (I am using the Mac virtualization) and transfer by SD cloud are very convenient. Only thing bothering was the iPhone not being quiet on mobile net. Yes, screenis small, but sufficient.
So, after a while I changed to iPad Mini, but keep the iPhone as backup, usually in flight mode on warm standby. If there would be a chance to get Jeppesen FD (still in use because of some things not available on SD) on an iPhone, I would switch to iPhone 6 Plus, but this way I keep it on the iPad for this season and dump Jeppesen next autumn, if it is still this way and go completely SD - ditching some possible locations.