Ok, let's be honest right there. some months or even weeks ago, this question was stupid for a lot of people. But right, it doesn't looks that mad, well to be honest it really can go either way.

The PS4 has sold over 60.4 million units worldwide back in June 2017, and should be close to 70 million units right now. PS4 total shipments are 67.5 million by the end of September 2017, and Sony expect to ship 79 million by the end of March 2018. A number which could be surpassed very easy to be fair, considering how PS4 has sold better than what Sony expected many  times (they indeed expected 78 million back in April 2017).

The Nintendo Switch has shipped a total of 7.63 million worldwide back in September 2017, is so far selling less than PS4 and Wii in the same timeline, but withouth any holidays. Nintendo expected to ship 14 million during the FY2017/2018 (Apr 2017-Mar 2018), which probabily mean they expected 14 million by the end of 2017. if this happen, Switch will be head of PS4 launch allinied for the first 10 months, and will also be the first time Switch and PS4 will have a "fair" comparation, because will include an holiday period for both platforms. This of course withouth assuming that Nintendo will surpass again their expectation, as is already happen (from the original 10 million prediction to 14 million)

As for the future, a recent rumor implies that Nintendo plan to ship more than 25-30 million Switch the next FY If this happen, then Switch will reach the crazy peak of Nintendo DS, which shipped over 30 million in the FY 2007/2008 and 2008/2009, and will also surpass by a huge margin PS4 peak year, which is 20 million in the FY 2016/2017.

So what do you think? Personally, i think that in the recent years i underestimated both console (Switch at the time knowing as the "NX"), i indeed didn't expect both NX and PS4 to reach Wii sales level in 2015, something which right now looks like the absolutely worst scenario. Both Switch and PS4 are expected to become "S tier" console in term of sales, so the question is, which one will sell more?

Tell me your opinion, and also write your prediction.

MY PREDICTION (As for November 13 2017)

NSW - 125m
PS4 - 120m

Poll as for November 13 2017

Will be update in future...

EDIT 23 November 2018

My predictions
NSW - 120m
PS4 - 130m

Pool:

EDIT 27 NOVEMBER 2019

NSW - 125m
PS4 - 125m

Pool:

EDIT 2 DECEMBER 2020

NSW - 170m
PS4 - 125m

PS4, world wide is too good.

PS4 should sale better. The only chance I see that the Switch beats the PS4 is an unexpected launch of the PS5 in 2018. But why would Sony do that as long as the PS4 sales like hot cakes.

I'll break it down by region:
North America: Switch by a small margin (I know this is bold, but the Switch is about to win 4 npd's in a row. If they can keep the momentum, it could happen. Plus, they have Pokemon)
Europe: PS4 by huge margin (shouldnt have to explain myself here)
Japan: Switch by a huge margin (again, shouldnt have to explain myself here either)

WW: PS4 will get 130M, while Switch will get 105M. Judging off the predictions above, it looks like a close race, but PS have a bigger audience in the rest of the world than Nintendo.

Didn't the PS4 sell about 15M in it's first year on the market? And Switch is expected to ship 16.7 by it's first year? They'll both be close when it comes to first year aligned.

The safe bet is the PS4. The bet I want to make is Switch. Hm..... I don't know

Too early to tell, but I would say Switch and PS4 will both be in the S-Club at some point in their lifetimes.

Wii was the ultimate party system. With the Switch, there truly ain’t no party like an S-Club party.

I don't think I could say yet, which just makes it all the more interesting.

Even if Switch outsold PS4 by 10% every week for the next 3 years, PS4 would still top it out by a large margin at the end of the generation. Plus, Sony has a tendency to support their consoles long after their time is done.

The point is, for the next 3 years Switch is not gonna sell 10% more than PS4. More like 100% to 300% more lol