Just had a look on both links and the reason is that one seems to be a B8 version, the latest and the other is a B7 which is no longer produced by audi and even the B8 has currently undergone a facelift so the new 2012 models look even more different.
As for your original qestion, the S line and Black editions both feature the same bodykit styling but the black edition trim level has the bang & olufson sound system with the chrome strip surrounds of the the S line on the foglights, grill and side windows are replaced with a black trim and the interior receives piano black inlays.
You also get those bright Xenon headlights on the S line and Black editions as standard in conjunction with sports seats in the front and of course, the S line badges around the car.
As far as i am aware, you also get the option of two 19" rim styles on the pure black editions including the titanium rotors (don't suit all colours IMO and look dirty quickly) or the 5 segment (which suit all colours but i'm biased as we have those
).
On the other trim levels, you receive equipment changes but to be honest, your best bet is to look on the audi website for the A4 brochures if your buying new or to get an idea of the sort of equipment that comes as standard on each trim level. Other things you may need to spec that don't come as standard on any trim level such as full leather.
On your last point of the Audi or BMW, it all boils down to which car you would prefer for your needs. The audi seems to have better spec as standard and the options from what you choose seem to be cheaper in places. You also get a better boot space on the saloons.
The BMW seems to be classed more of a drivers car with better power, MPG and handling but minus the above. So it really comes down to which things you prefer having if your buying new OR whether you find a great spec car in your budget from either manufacturer, IF you aren't bothered about running costs on either which should be close to each other for the most part.
