Why even go with a '05 ninja? Why not go for an earlier model even? I have a 94 Ninja to learn from, after which I plan to get something bigger; but if you're just going to learn on the bike for a year or two...get something even older and save a little bit more.
Both bikes are good for a starter.the best thing for you to do is go sit on both bikes and feel witch one feels better for you.i would go with the gs500f if you are going to do allot of highway riding.if you do allot of back road and city riding i would go with the 250r.