Not really a fan of doing memory improvement training to improve memory. I would go for the more organic and fun way of exposing a kid to as much novelty (the good ones) as possible. The more novelty your child is exposed to, the better his or her memory becomes as well. Plus, he or she gets to...
I would pick the piano. Less physically demanding for a kid as it requires less precise finger placement and less physical strength, and more versatile.