It's definitely not a Super Mario 64/Super Mario 64 DS scenario, that's for sure.
The DS version of Super Mario 64 added so much, and all for only $30! 3 extra characters, 30 more stars(Including a handful of new levels to boot), some stars being fixed like the caged star in Whomp's Fortress... I could go on.
Then we have the Diddy Kong Racing DS port and holy crap... If one already owns the original, there is absolutely no point in having the DS one(Outside of mostly dead online and wish tracks that weren't fully fleshed out, and are a pain to unlock anyways).
The DS port of Diddy Kong Racing adds so much useless crap, all the while taking away so many minor and major elements. Sure, Wizpig & Taj are now playable, but at what cost?!
I could go on, but I'll let the link speak for itself. For every positive change in the DS port, there's at least a handful of new negatives that pop up as a result.
If you already own the N64 original, cart only or complete, then do yourself a favor and skip out on the DS port unless you're buying it as a gift for someone who doesn't own a N64 and the original cartridge.
(Huh, this wouldn't make a bad GameFAQs review actually.)