I think whether PS4 or Switch sells more in 2018, it will come to a gap of less than 500k, possibly even something like 100-200k. It's a total coinflip that depends on deals at retailers to the largest extent. $299 and PS4 loses IMHO. Widely available $249 bundles and $199 BF deals, PS4 probably takes it, as I have zero expectations of any bargains on Switch beyond some limited $299 and $329 bundles.
What does seem extremely likely, and far more interesting to ponder, is that I think Nintendo will miss the 20M global sales by 1.5M or so, while PS4 will beat their 16M expectations by 2-2.5M. So they might globally nearly tie as well, but investors would have the opposite reactions to this virtual tie.
It will be fascinating to see. Looking back at 3DS Pokemon and Smash, systems which were dramatically more affordable than Switch, sales were great but not world shattering, so I'm not terribly confident that $300+$60 is going to translate into world beating sales this holiday. Surely excellent sales, but enough to cover a 400-600k gap? In the end, depends on how things are priced and available imho.