Based on years of reading about both schools, I would say the overall academic vibes at the schools are:
Brown: Laid back; intellectual, but relatively less hard work, less intense. You can work hard and get an A, or take it a bit easier and still get an A.
Chicago: Intellectual, intense, hard work. You will work very hard for an A.
There are pros and cons to both vibes. Having to work harder at Chicago probably means more stress while you are in school, but it’s likely beneficial down the road: it will prepare you to have to dig deep and persevere later in life.
If you enjoy working hard/studying hard, enjoy discussing academic concepts and love a tough challenge, you might be the type of person who can take full advantage of a UChicago education and perhaps even enjoy it.
If you’d rather take it easier, relatively speaking, while still having the ability to grind if you want to, Brown might make more sense in terms of academic vibe.
Another obvious deciding point is going to be the cities.
Last thing: I would avoid applying ED if you are uncertain about which of them is your #1 choice.