Xbox One even if it's a tough challenge to accomplish.

Xbox One.

XB1 for sure. M$ will do whatever it takes to be in second place by december.

Let's assume the 10 million mark would be crossed by one or the other by the end of the year.

Nintendo has sell a little over 4 million units of hardware in 10 months.
Microsoft has to sell a little over 6.5 million units of hardware in 10 months.

During a similar timespan last year, Nintendo sold 2 million hardware units of Wii U. During much of the summer, Wii U was scraping by with about 20k weekly worldwide. With MK8 fresh on the shelves and recent releases like 3D World, Donkey Kong and other games SSB on the horizon, is it impractical to assume Nintendo could at least double what are considered horrible sales? No. That's only going "really bad" to "slightly less bad" and within the realm of ration possibility.

For XOne to sell at the 650k/month pace, it would have to match or outsell what the PS4 is currently doing. How likely is that going to happen?

I'm going with U. Smash and Mario Kart will push it over before Xbox reaches 10 million.

The WiiU will definitely hit 10m first, of course it helps greatly that it has a one year lead. Had all three console released at the same time the WiiUs sales would be even more sluggish then they are. I'm curious to see if the PS4 will pass 10m by end of the year.

I am thinking wiiU just cause it has more exclusive games in 2014 than x1 as of this moment and TitanFall is available on PC/360 but we will see

The obvious answer is that neither of them will break 10 million...

I'm just kidding :p I think XBO will hit the mark before Wii U. Titanfall will give a bit of a boost to XBO and help steady its weekly sales. SSB and MK8 will give Wii U a great push in the Summer but once the holiday period rolls around there won't be any further demand for Wii U and XBO will have good demand in the USA. Halo is the biggest console exclusive on any platform now. Even the prospect of the game gives confidence in the product.

Titanfall and Halo this year. Probably another Halo and god knows what more next year, plus 3rd party games that Wii U misses i think we have a winner

Xbox One. I think it will be close, but Microsoft will cut the price of the regular console in all territories this year, something Nintendo said they weren't going to do until 2015 (although they may be forced to if MK8 or Smash doesn't improve sales)