I go with a single card (Citi PM), because it simple and I only have to carry around a single card with me. Not that the other schemes are beyond my intelligence they just take to much time and effort for the return I would get out of them. Citi PM does have slightly less earn but on the flipside a better range of partners, some good promos from time to time, and online banking.

Also the extra bonus from running multiple cards tends to be defrayed by points transfer fees from some cards.