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.