I think how much they will expect you to know about fleet details will be commensurate with your flight experience/ exposure. Knowing the number of aircraft in each fleet is not too big a deal - thought the mergers are going to complicate things in that department considreably for a while!
In terms of why an airline might select Boeing over Airbus or vice-versa, a great deal is about politics - ie. American airlines supporting American companies like Boeing, and European airlines supporting Airbus. They will not expect you to know interface differences between aircraft - Basics such as Airbus being Fly-by-wire is one thing you might want to mention. Also you see all airbus aircraft have winglets and now the 737NGs are doing this - why? (aerodynamic efficiencies from wake vorticies). I would concern yourself more with knowing about what you are actually going to be doing as a pilot on a day to day basis. Also, get together loads of examples showing how you have demonstrated different core qualities.