It's good to see an educated buyer and fairly comparing apple to apples only.
Though both of them are in the same league and offers the same feature, performance, and pricing so I really like what desertdude said that drive both and decide which make you smile more.
I agree Audi is more stylish than Merc, but Merc has more elegance than Audi and few people like that over style.
Research more on what Stig said about DSG, then you will reach your conclusion faster. Also, you can search and find more of known problems and recalls happen in either model like this one
Either way, congrats in advance for your next ride and do share her pics whenever you get it.
Our resident mech Saleem is not in favour of anything with a bit of style and not made in Japan, surprised he didnt try to change your mind to go for a corolla or sunny instead so take his advice with a grain of salt.
IMO Merc would be a better choice since its more common and more bits and bobs including after market available for the Merc, also generally Mercs are said to be more reliable than Audis.
Although A4 is a better looking and more snazzier car IMO at the end of the day. Get what make you happy, you only live once.
Test drive a few examples of both and see which one you like better and go for that. What would you rather have a smile on your face everyday or a few extra coins in your pocket.