Conn 6d(1975) or Yamaha 644?

They are both good instruments for a 10 year old. My personal preference would probably be for the Yamaha. The 664 model is discontinued but regarded by some players to be one of the better models Yamaha built and should not have discontinued. (Dale Clevenger liked them I think...) They would both be good horns but I think the Yamaha might last him longer if he becomes serious about music. Also I echo the sentiments about eBay.