It will be Nintendo, for sure (shipping that is), in NA/EU as NoA/NoE has to clear their warehouse stocks of the 3DS(XL) model by early 2015. Expect a price cut and a lot of shipped units.

The 3DS sold 3.3m units last December. For it to win 2014 it needs to match those 2013 sales, needs the PS4 to not widen the current gap, and then for the PS4 to sell only 1.6m units in December (for context, the PS3 managed almost 1.2m units last year, its 8th December).

I'd certainly put the odds in the PS4's favor :p

Why? 3DS/XL production and sales will not stop when the New 3DS comes out, so why would they need to get rid of stock?

Then that's probably the answer right there. 

I see no reason why the 3DS would sell significantly more units in 2014 than it did in 2013.

Someone mentioned product dumping, but this is Nintendo. Their business model remains based upon generating profit on every unit sold, so there's not going to be any ridiculous manufacturer's discounts to clear out a product line. 

It pretty much goes without saying that the PS4 will move more than 1.6m units in December unless SCE completely fumbles production and distribution heading into that month.

Wut? 

Except they have shipped like 1/3 of what VGC shows they have sold this year up to June...

No, we already went over this. I'm not doing it again, sorry.

3DS will sell more. Nintendo owns Xmas.

No it doesn't. Will happily remind you in 3 months time

Yeah DS is not going to win Xmas, the PS4 will. In fact it's going to be massive.

PS4, duh.