VERY different. The difference is that Flatbush is big enough to support its own stores, which cater to its misnagdish community. Monsey has pretty much one main Jewish shopping district, the 306/59 area, where EVERYONE goes (and I mean everyone. Like I said, WK is NOT enough). There is one Costco where EVERYONE goes. There is the Palisades Mall where EVERYONE goes. There are a few isolated shopping centers, like the WK one and that one on Saddle River Road, but those are not enough to survive on, and like aspiringrabbi said, chassidim own most of the best places to shop around here.
On any given day, you may not feel it as much. You might buy a nice house on Sherri Lane or McNamara Road, go to a nice litvishe shul, have nice litvishe neighbors, and hop over to WK to get a loaf of bread- but if you want to do anything more interesting than that, like drive your kid to practically any school except YSV Girls, Bas Mikra or Chofetz Chaim, or eat anywhere except Al Di La or Bubba’s, or any of the thousand and one things most people need to do, you’ll have to bump into chassidim.
It depends on whether that’s a dealbreaker for you. If it is, I gotta say that that surprises me a bit, but that’s your business, not mine.