Does meditation improve working memory more than exercise?
Focused attention meditation practices improve focus in the long term. So the improvement is difficult to asses and could probably only be found in a standardized test setting. Physical exercise however has been demonstrated to improve mental abilities in the short term (after recovery of physical exercise) levels of circulating neurotransmitter stay elevated for approx. 48h.

http://online.liebertpub.com/doi/abs/10.1089/acm.2007.7022 (Meditation and focused attention)

http://www.sciencedirect.com/science/article/pii/S0001691802001348 (Physical exercise and cognition)