Designing an ios app

I am about to design an IOS 8 app for the very first time. I've been a designer for 5 years now, both on Photoshop and Illustrator, but I never got the change to design an app. I am looking for some guidelines. My questions are: Which program should I use? Photoshop or Illustrator? I am much more comfortable on Photoshop, but I am afraid of the sizing. Which brings me to my next question, which canvas size to pick, iPhone 4, 5, 6 or 6+? Or maybe a design for each size (same style though). Thanks!