For this one definitely core graphics and core animation. Jeff LeMarche has a sample template for a Black Jack game on his blog - I just finished a suduko puzzle this approach. As far as cocos2D which I am using for a platform game - openGL would not improve or enhance the graphics - it will definitely not make the graphics look better - you can photoshop your heart out on this one and feel safe and know it will look great. Learning Cocos2D for this is too much overhead - same with learning openGL ES - just use core graphics and get it out there. As far as knowing openGL ES for the future - of course - but you should understand Apple's "core" before attempting these.
