I'd get the largest capacity filter that you can afford especially with large fish. Remember that the flow rate on the filters are under "ideal" situations and the published numbers assume you are not putting things like Co2 reactors on them. While I think you can get by with the smaller filter why not invest a little extra into the bigger filter that way there you don't have to wonder if the filtration is adequate enough and you can always use it on a larger tank if you ever decide to upgrade.
IMO Discus are too big for a planted 75G tank. Unplanted is another story though...but that's no fun.
I'd go with smaller fish like the Angels.