Literally just bought a 2011 S5 in Texas this past weekend. It was in Austin. At the Audi dealership there. Nice people. Drove it home to Chicago over the weekend. Anyway, IMO get the following:

Prestige Package
Drive Select w/ Sports Rear Differential
B&O sound (believe to be part of prestige but if not, get it)
Advanced Key (again, might be bundled in with Prestige but if not, nice option)

BUY A VERY GOOD RADAR DETECTOR. You will attract attention, for better and worse.

Drive it like you stole it.

Enjoy.

 

Drive Select is basically an option that allows you to alter the sensitivity of the following: steering, throttle, suspension, and rear differential. In each case, Audi describes the settings as: comfort, auto, and dynamic. Comfort will relax the steering so as not to be too darty and provide more on-center forgiveness, throttle will be less sensitive to input, suspension is more compliant, and the rear diff... actually don't know what happens here but you get the idea (comfort = relaxed). Auto will self adjust settings to your driving style, so leave it in auto and it will decide for you what you need (it is also the default setting each time you turn the car on). Dynamic will tighten everything up and give you the most sensitive, taut setup that the car has to offer.

Sports Rear Differential is a torque vectoring rear differential system that alters how much power is going to each of the rear wheels. The best way to describe how it works is to take a styrofoam cup, lay it on its side and roll it. Due to each end of the cup having a different circumference the cup will not travel in straight line but rather will turn. By having differing power at each rear wheel, essentially replicates this circumference example. The car will turn-in quicker. People will say it helps make the car feel more like a rear-wheel car, I don't know about that but in theory, the car will rotate faster with this option. There are plenty of reviews of the system and the car with varying degrees of explanation. IMO, the sports rear differential is most apparent when driving at 8/10's and above. Below that, can't say I notice anything at all.

Lastly, you should know that the two options described above came bundled together for at least a couple of model years. Mine is a 2011 and they were bundled together. A new, 2013 S5 has each as stand-alone options.

Hope this helps.