Q. I am planning to buy BMW 3 GT or Audi A6?

If you are driving yourself then you will enjoy the GT, Audi A6 is a good contender but a tamer traditional choice. I picked the GT over the 3 series because the rear legroom (as I had a chauffeur then) it had more legroom than the 5 also. The only trick which becomes evident later is that the legroom comes at the cost of a smaller seat base. So if you are under 5'7" you wouldnt have nothing to complain about, but if you are a larger frame then you are better off in the front seats. Driving pleasure, fuel economy, air conditioning etc are above par in the GT. Running costs I dont know much because I have a complete service package which takes care of everything.

BMW 3 GT

Audi a6