I agree with some of the othe posters here - using a vector drawing tool such as Illustrator for designing an interface is much easier than using a bitmap tool like Photoshop (although Photoshop has some vector drawing tools). The main difference comes from the ease with which you can simply select, drag and re-size elements on your screen compared to Photoshop. This will definitely help you work faster if you are experimenting with layout.

You can use a vector drawing app for wireframes, for mapping out screen flows and interaction behaviour, and finally for creating the finished visuals for the app. Photoshop is only really suitable for the last of these tasks. There are some visual effects that only Photoshop can achieve, so you may find you have to use both. But I would always recommend starting with a vector drawing app.

If you don't want to use Illustrator (which has a clunky UI in my view) there are plenty of cheaper vector alternatives . Here are a few

Inkscape (open source and capable of professional results) http://www.inkscape.org/en/

Sketch (Mac only, praised for its ease-of-use) http://www.bohemiancoding.com/sketch/

Serif DrawPlus (Windows only, low-cost but very capable) http://www.serif.com/drawplus/ (no trial version, but they are selling the previous version for £10 (approx $16 usd): http://www.serif.com/promo/

Xara Photo and Graphic Designer (Windows only, again low-cost but very fast, capable and an excellent interface) http://www.xara.com/uk/photo-graphic-designer/

Edit: just to add, the apps above do not have anywhere near the amount of tutorials for learning as Illustrator (or the industry recognition), so factor that in when choosing a vector app if that's important to you.