I applied to both. I got the intial email from NW, but then i also got invited about 1-2 weeks later. Got invited to U of Chi too. I think U of Chi has offered a decent amount of their interviews (~50%???) but I think NW has offered very few thus far. (~20%???).
In my invite from NW they asked me to schedule my interview in the first 2 weeks of november. If they are waiting for the MSPE, which is released Nov 1, it would be difficult to arrange time off and book travel those first 2 weeks, so I think they offer just enough interviews to fill up some of those interview days.
"Our interviews generally begin in November after we have received Dean's Letters. We do, however, have the ability to schedule interviews earlier in the season for those applicants who have been positively reviewed. We would strongly encourage you to schedule an interview with our program during the first two weeks of November"
They interview thru late january. So in order to offer enough spots to completely fill the first 2 weeks (or 1/6 of the interview season) they would theoretically need to offer 17% of their interviews pre-MSPE.
This is all conjecture, but hopefully it gives you a realistic picture.
Good Luck!!