Illustrator is a vector-based program, in which everything is mapped out using points. It's good, for really crisp clean lines, as when you scale it bigger, there is no pixelated distortion with what you make in the program. Photoshop is a program that works on rasterisation, which is in pixels, and each pixel can be manipulated individually. It''s pretty much comparing apples to oranges here. It's more about personal preference, and what exactly you want to do.