3DS of course

I pick the 3DS.

It's guaranteed that the 3DS will sell over 15 millions this fiscal year. But I don't see the Vita selling 6 millions in the same period. This could be possible but I don't see it. Even the pricedrop had little effects in Japan.

The Vita is probably going to end its life at around 18-20 millions in some years.

You know what... I could actually see Vita doing close to 10m NEXT year.... Or the year after that. PS4 could potentially boost the Vita numbers up towards 10m units/year. I don't think it's out of the question that Vita will do 10m one year, but not this one. Maybe it'll 5m....

Vita will never sell 10 millions a year.

I wouldn't bet for or against it. I think it kinda goes hand in hand with how well PS4 does, and what sort of connectivity between them. What if it gets bundled with the PS4? How about then?

It has nothing to do with the PS4. The GBA connection did not help the Gamecube. PS4 owners are not going to buy a Vita because it's from the same company. And a console bundle would make Sony loses ton of money.

As I said, I expect the Vita to do a little less than the GC lifetime.

Well, I don't really think the connection between GBA and GC will be anything like Vita/PS4. Wouldn't it be neat if you could play your PS4 games on Vita? I would think that connectivity alone could push a few Vita....

They will hit it at about the same time in December 2013/ January 2014

Well PSP seemed to benefit from PS2 initially 

Vita didn't sell 4 million in its first year and I'm not sold on PS4 connectivity. So I'll go ahead and say 3DS, easily.