If I may suggest a 3rd alternative...
If FinalCut Pro is out of budget or you don't feel it's justified getting it when you're not taking advantage of all it's potential for youtube videos and it's just harder to use
and if iMovie is super nice, free and easy to use but you definitely know its very restricted in capabilities
then may I suggest Screenflow. I've used it for all my videos since I started getting serious enough about YouTube. It's only $100 (which compared to final cut and adobe software is very cheap), but what I love about it is how use friendly and easy & efficient it is to use for someone who doesn't have years of experience with editing software. At the same time however, it's functionality is much much greater then that of iMovie. The only reasons I would consider upgrading so far is if I wanted to do serious color grading, involved special effects & animations, or I wanted audio functionality as extensive as Logic or something. Otherwise for everything else it's honestly great and easy to use. Also has a built in screen/webcam recorder.
Food for thought.
I just feel it doesn't justify the average YouTube choosing between such contrasting software when there's a more suitable middle ground.