Either is fine.
PS and AI use two completely different and UNRELATED approaches to graphics. PS is akin to a mosaic.. moving and paint and image based on lots of tiny tiles ( pixels) AI is a graphic interface for formal based drawings.. you never see the math but its all math... YAY MATH!! Where as in photoshop you will need to keep in mind the ACTUAL dimensions of your images. you can scale vectors (AI) to any size.
in short: PS is for 'photos'. and AI is for 'drawings' .. which one appeals to you.
Adobe has made the interface of both AI and PS very similar. So after you are finish with one you will naturally have a familiarity with the BASICS of the other's interface.