There are pros and cons to both.
Audi
Pro :
Generally more powerful than the Merc.
Cheaper to buy, run and dispose.
Good fuel consumption.
Less harsh ride.
Cons:
Not many standard options.
Options can add a big increase to the price.
Generic looking and feels like an upscale VW Golf.
Some cheap looking bits.
Discounts kind of hard to come by.
Mercedes
Pros:
Futuristic and unique styling.
Refined finishing.
Exclusivity.
Lots of standard options and options don't cost as much as one would think.
Fairly easy to get good financing and offers.
Cons:
Expensive to buy. Servicing won't be cheap.
Harsh ride especially those with the sports suspension.
Fuel consumption not that fantastic.
Most likely will be asked if it is mum's car
Probably won't buy either of it.
Prefer an Alfa Romeo Giulietta for a fun car to drive, a Renault Megane RS for outright performance and a Volvo V40 for a nice everyday car.