The software you want to use hinges largely on how you would like to make the cards, what kind of artist you are, and what skills you want to use:
- Do you like creating pictures that look like paintings, or combining photos and text like in a movie poster? Then you could make your cards in Photoshop.
- Do you like creating modern-style graphic art, drawing cartoon-like characters or subjects that tend to have distinct shapes and hard edges, or working with patterns and line styles that are easy to edit later? Then Adobe Illustrator might be best.
- Do you expect to create the cards by combining clip art or stock photos, maybe because you’re more into collaging rather than painting or drawing by hand? Or you want to make a template so that it’s easy to create different cards based on the same design, by changing a photo or other picture, and changing the text? Then InDesign could be the one to use.
I’ve created greeting cards in all three of those applications at one time or another. All of them let you mix different kinds of imported art, create high-quality typography; structure your design with layers; combine layers by using opacity, masks, and blending modes; and export a press-ready PDF file to send to the company printing the cards.
The differences among the three applications come down to what they focus on, which is what that list is about. Photoshop and Illustrator are great if you are skilled in painting or drawing your own art by hand, InDesign is more about combining photos and art from different sources and typography into a unified layout.