I was in the same situation as you a few weeks ago (a new switcher and a hobby video editor trying to learn Final Cut/iMovie), and ended up buying the high-end 13 MacBook Pro (specs in my signature). The best computer I have ever owned. Editing in iMovie is a snap, no slowing down whatsoever. I haven't tried Final Cut Pro yet, but I expect good results 😀
As this is your first Mac, I suggest you not to go for an expensive Mac like the 15 MacBook Pro or the retina MacBook Pro, because you do not know how much power you exactly need, so buying an expensive Mac could be an overkill. You can upgrade the Hard Drive (or SSD) and the RAM yourself on the 13 MBP (very easy procedure, do a search on YouTube), and those prices are typically much cheaper than what Apple charges for upgrades. As for the MacBook Air vs MacBook Pros, the Pro have more power for tasks like video editing at the same price point, so the Pro wins, unless you really want a super thin/light notebook. Overall, I think the high-end 13 MacBook Pro is the sweet spot.