What is the best tool for creating simple iOS demo videos
If you haven't used iMovie in a few years, I recommend you give it a try. It does exactly what you're looking for. If you create a new movie rather than a trailer, it will just create a timeline that you can drop footage into. On the left, under your events and library is a section called "Titles" that has dozens of titles ranging from tasteful to playful. You use the "V" key to record a voiceover.

FCPX is also a great easy tool to learn. In fact, iMovie is now made so that you can transition easily from using iMovie into using Final Cut Pro X. I think they might even share some of their internal bits. In any event, if you use one, you can probably do some stuff easily in the other.