You won’t be disappointed with either engine. I have pasted in below a few magazine quotes regarding the TDI that you may find of interest.

CAR AND DRIVER - JUNE 2014

"One Quick Diesel - ... even in Quattro guise and with taller final-drive gearing, the diesel clobbers the gas car to 60 mph, hitting the mark in 5.1 seconds, a half-second quicker than the last A6 3.0T we tested. ... Through the quarter-mile, the diesel holds the advantage with a 13.9-second elapsed time compared with the gas car’s 14 flat. But the 3.0T’s 101-mph trap speed is 3 mph higher than the TDI’s, showing how the gas car’s wider power band allows it to reel in the diesel once they’re under way."

"Its smooth, immediate torque delivery and the eight-speed automatic’s snappy shifts make the diesel an easy way to slither through traffic."

"Or maybe you’d rather surprise yourself and use this diesel sedan for a satisfying back-road burn. Its lag-free power delivery nicely complements the A6’s imperturbable feel and sense of unmatched solidity, producing a car that does what it’s asked, when it’s asked." 2014 Audi A6 TDI Diesel Test ? Review ? Car and Driver

FORBES - MARCH 2014

"Turbocharging matches well with the diesel engine’s performance characteristics, and the A6 is tuned to deliver torque throughout its rev range, with no perceptible “turbo lag” in the process. The eight-speed Tiptronic automatic transmission is smooth as silk, and keeps the engine in the right range to take full advantage of the diesel power."

"Put into even simpler terms, the A6 TDI just feels like it charges down the road, pushing you back into your seat and flattening your eyeballs into your skull."

"The A6 TDI is the first diesel-powered sedan that I’d actually buy, and would recommend to my premium-inclined friends. Drive one before you settle for a gasoline-powered sedan, and I think you’ll be surprised and intrigued." 2014 Audi A6 TDI Test Drive And Review: Diesel Does It - Forbes

AUTOWEEK - FEBRUARY 2014

"The diesel makes it more appealing to me. Why? To my mind, it has all the diesel benefits like torque (check out that 428 lb-ft!) and fuel economy without the diesel drawbacks including smell and coarse operation (I couldn't tell I was in a diesel from the sound).
The car leaves the line fast and smooth. Throttle response is right now, unlike some competitors' diesels, and the power stays stout throughout the revs. The overall diesel smoothness and operation make me wonder why anyone would order it with any other engine. (I don't include S model in that view because that's another kettle of fish entirely.)"

"This 2014 Audi A6 TDI is an excellent car. No, it's not cheap, but excellence costs money. The car is worth it." 2014 Audi A6 TDI Prestige review notes Autoweek