It is an interesting question. Personally, I see Nintendo keeping with the SD Card route...and personally, I like that method best.
Yes, SD cards aren't as fast or as cheap as a hard drive, but they do offer some benefits.
I like the idea of being able to switch out SD cards for certain games and such. Say if I had Rock Band I could have and entire SD card devoted to my rock band saves and music files...then whenever I played rock band or took it to my friends house I just bring my one SD card.
Another SD card could be solely used for in game saves.
Another for arcade game downloads and such.
Another for Apps that are compatible with the Wiitab on the go. (If it has an SD card out.)
The point is, SD cards open up options I like, and I hope Nintendo continues going that direction. Though I hope if they do, Nintendo allows us to decide where games save. Default being the internal memory, but an option to save direction to the SD card...and play off the SD card.
As for internal memory. I think Nintendo should go the iPad route. It only needs about 32-64 GB SSD memory...and then the SD card support, and that is something Nintendo should be able to put in at its desired price point.