In practical terms, the advantage of a #8 is when you are match planing thick board edges, but the #7 offers more benefits than the #8 most of the time. The frogs are interchangeable with the 4 1/2 and 5 1/2, it is wide enough and long enough for most jointing needs and works excellently on a shooting board. It's also lighter than #8.
I love my #7 but really only use it for edge planing. A 5 1/2 is my preferred plane for the final jointing of faces, and a jack plane for the rough preliminary work. Using a #7 for jointing faces is very tiring, and if you have winding sticks and a good straight edge, you don't need to rely on the length of a #7 to achieve a flat face.