I've pointed it out before, but comparing 3DS to DS is a bit tricky.
The market has changed, 2012 is not the same as 2004.
Two screens is nothing new by now, touchscreen interfaces have been optimized well beyond what DS or 3DS offer (although I have to say 3DS' touchscreen is more than fine).
Console-like experiences on handhelds are nothing innovative anymore, even though it's still a tricky market for some franchises (good budget management is needed).
All in all, i think 3DS is doing quite well for itself, even if it's not setting the charts on fire like DS. At least for the moment it's selling better, but we all know that will change pretty soon.
Same could be said for Vita, well.. if it wasn't so abysmal. I think Sony will have a dramatic change from last gen to this one, in where sales in the Americas+EUR will far surpass Japan. That is unless things are corrected.