So we are either looking at a huge fall for DQps4 or a huge boost for the squids, or perhaps a bit of both
Well, let's see (of course, GM isn't a perfect indicator):
01./01. [3DS] Dragon Quest XI: Echoes of an Elusive Age # <RPG> (Square Enix) {2017.07.29} (¥5.980) - 307.714 / 1.456.601 (-73%)
02./02. [PS4] Dragon Quest XI: Echoes of an Elusive Age # <RPG> (Square Enix) {2017.07.29} (¥8.980) - 200.798 / 1.151.113 (-79%)
03./03. [NSW] Splatoon 2 # <ACT> (Nintendo) {2017.07.21} (¥5.980) - 74.235 / 827.646 (-30%)
Splatoon 2 is stock-straddled for its sales figures. So assuming we don't see ~200k hardware, its unlikely these are the numbers we're going to see next week. Since I expect 80-100k hardware for the Switch with the arrival of Obon, I'd expect ~100-120k Splatoon sales.
So DQXI-PS4 would be under that figure, DQXI-3DS would be above that figure which makes sense, the casual audience exists in an order of magnitude more on the 3DS than either the PS4 or Switch combined. PS4 legs should be expected to be the shortest, its mid-life hardware, its total population is low, and its a front-loaded software console most of the time.