PS is really heavy. It's powerful, but cumbersome for app design. Indd is way faster if you're designing up flows. You can change a color universally, embed things in different files in one command, you can set styles for type so every header, subhead, paragraph, etc, is the same, and you can change that all at once across all you files/design. And if you make a style guide, it's a lot faster.

I'm just an amateur, but I like to design as much as possible in vector graphics (I use inkscape) and then switch to bitmap (gimp for me, photoshop for most I guess) for the finishing touches. Makes it so much easier to scale stuff.