I’m biased: my D is at Brown and I like it more than I like Penn, largely because I like College Hill much more than I like West Philadelphia, and I like Providence overall more than Philly.
With that said:
I have no idea which one is harder and whatever difference there is in that area is probably unimportant. The ranking and rigor difference between these schools is negligible, and I wonder how anyone can really assess whether one is harder than the other with any real degree of precision. But maybe Penn is harder. If you believe that, and it matters to you, then there you go.
The winters in Providence are no worse than in Philly; not sure where you got that impression. In fact, in my time there, winters have been great. Providence is right near the ocean, which has a moderating effect on weather.
There is plenty of partying at Brown.
Good luck. Can’t go wrong with those choices IMO.