They are quite different styles. I have owned both (Electra 926 and B&W N804) and kept the Focals. The B&Ws in my opinion have a slightly dry treble and lean midrange that the Focals don't have. If you like strict detail, then it is very hard to beat the B&Ws but if you prefer coherence and body, the Focals are the ones for you. The Focals are also a bit easier to drive and sound sublime with tube amps. The 804S is an improvement over the N804 (my neighbor has a pair of 804S so I know them well) but it still has that signature upper mid peak that I just don't really care for. And finally, even the Focal 918s have bigger and fuller bass than the 804S so the 928s will load a room much better than the 804S ever will. The 804S require a fairly small room to be tonally balanced - something on the order of 225 sqft. So if you like bass and big soundstaging, the 928 is definitely the better option.
Arthur
Arthur