Honestly, it doesn't seem to be that huge of a difference. Sure, the HD is much better detailed for the obvious reasons, but when compared, the 3DS seems to sit next to the HD one just fine.
Also, this was one of the best Streetpass games so far, and it is one that I have yet to turn off because I keep getting great rewards to Streetpassing. Obviously, the HD one will not feature Streetpass, but I wonder if they'll do something equivalent.
"I've got better things to do tonight than die." -Springer