Take a look at the various photos in this thread and you'll be able to see for yourself. In summary, the differences on the exterior are with the front bumper, front grille, rear bumper, side skirts, and a more pronounced integrated trunk spoiler on the S5.
Also, note that the A5 with the S-line option comes with side skirts, has the same rear bumper cover as the S5 (but with a different rear valance/diffuser), and the same front bumper (but bottom center piece is not silver color like on the S5). See this thread for more details.
And of course, with the A5 S-line package (or the S-line exterior kit in countries where the interior and exterior packages are separate), the front and rear bumpers and side sills are the same as the S5, so that the only body difference is a slightly more pronounced integrated lip spoiler on the S5, and of course the S-style grille.