As a travelling marketing guy, here are my thoughts and experience...
Many of the YouTube vloggers use the £500 Canon cameras or the SONY / Panasonic ones. I've used the Canon's and they are super.
I can instantly tell when an iPhone video has been uploaded to YouTube. It always makes my heart drop as it seems to lack a certain sparkle or life which you get with a proper camera.
Even the ones from photographers that Tim Cook promoted the other day did not look good to my trained eyes the other day. I can tell within a split second. Whether the quality is dropping when edited in FCPX/Premiere, I don't know but a small camera still seems to make a more polished job - but you go to remember that the iPhone does around 100+ extra things - and in all fairness, it does most things really, really well.
Pro standard or YouTube vlogging, making film/docu's or client work - buy a £500+ camera.
Every day life, travelling videos etc, filming your friends & watching in the Photos app on your Mac, filming to upload to Instagram - buy an iPhone Pro.
Check out a few vloggers on YouTube - EverythingApplePro uses a Panasonic camera.
The Wolves (Adam) uses a £500 small Canon camera, edits on a MacBook Pro in FCPX AND his videos look very very clean with great picture quality. For audio, he's using a RODE mike.