I have been using Adobe Premiere for years. I used Final Cut Pro 7 a few times but could never really get into it. I started forcing myself to try out FCPX when it came out and now that I've gotten over the learning curve it is a lot easier to use. I've basically abandoned premiere at this point. As far as why is it better. There are a lot more plugins that you can use for X that don't work in iMovie (Red Giant are some of my favs that come to mind). Something else cool you can get is a keyboard cover
http://kbcovers.com/final-cut-pro-x-keyboard-cover/ that will help you will all the shortcuts. I use it on a Mid 2012 MacBook Pro (Retina) i7 and it can edit 1080p without a hiccup, a small slowdown on 4k however still manageable.