I chose both, and when we were reccommended to drop one from AS-> A2 i chose psychology.
Product design was fun, for As we made a lamp for courswork (and a large folder with it) and got A2 we made a radio. You design your own lamp/radio/whatever and then create it, which was always fun and a nice change from sitting in the classroom like you would with psychology. The product design exam (other 50%) was the hard part, learning about the different materials, plastics, metals, smart materials, infact the knowledge is vast but we were given textbooks and told to learn it, the actual exam when it came to it i found okay though.
basically i reccommend product design for the coursework part of it, its alot funner.