First of all, this is intended for people who have played both games, not just read reviews.  Second, I'm not doing a vote, I just want to hear people's opinions.

I prefer Diddy Kong because on WIFI, you can race up to 6 people, and you can specify the game type you want to play and the number of people you want to play against before you find random people.

I like the battle mode in Diddy Kong, but when people just sit on the shield balloon, it is no fun to play :-(  As for online, I like that you can use your friend's icons once you have played them in a match...

I really liked Super Mario Kart on SNES & DIddy Kong Racing on N64. With that said, I just felt that single player for Diddy Kong Racing DS lacked in some way, although I was cheerful seeing some familiar tracks. I guess I like Mario Kart better because the fact that you have people snaking and that means you have more of a challenge.  :? Viva La R.O.B.

If this was between MKDS and Diddy Kong Racing 64, I would say DKR, but DKRDS is horrible.  Microsoft (who now owns Rare) thought they could just throw it on the market with almost no work on it, and it shows.  The origianal DKR was really fun, though.