I have a 1.9 PD Skoda Superb, which probably has the same power unit as the car you are considering. I did some research before purchase.

The top gear ratios of the 5 and 6 speed transmissions equate to 33 and 35 mph per 1000 engine rpm respectively. This means that at 70 mph in each top gear, the engine is running 120 rpm more slowly with the 6 speed transmission - not significant.

The PD engine has such a slug of torque that even a heavy vehicle does not require a large number of gear ratios to make good progress. In a market where there is a 70 mph blanket speed limit, it makes no sense whatsoever.

The 6 speed transmission has some significant snags. It's far less reliable than the 5 speed unit and suffers pinion bearing failure. The addition of the 6th speed entails the loss of synchromesh on reverse. This is something I find very handy when towing and making multi point turns with the trailer - the makers don't mention synchomesh at all on either transmission. I discovered it by accident.

Finally, the 6 speed transmission usually comes with a co-axial clutch slave cylinder. Any problems mean transmission removal - expensive.

In the UK, a 5 speed unit is a far better bet.

659.

I owned a B5.5 Passat 'sport' with the 1.9 PD motor and 6-speed box for 5 years and 62,000 miles from new (same floorpan, drivetrain etc as the A6).

It was an excellent car and I found the gear ratios almost ideally suited to the power delivery and UK road conditions.

4th gear gave fantastic A-road overtaking punch with no need to change down: 40-60 and 50-70 in 4th gear were typically around 5 seconds flat, which just a few years ago used to be supercar territory. The trademark instant 'shove' of the PD motor made the car feel (if not sound) unexpectedly sporting. Even more so when using a tuning box.

6th gear gave 85mph and 2,650rpm, which was quite relaxed. Over the life of the car I averaged nearly 47mpg (measured by tankfuls against miles travelled) which is pretty good for a 1.5 ton family barge.

Never had any gearbox or clutch problems in my ownership, which included nearly 10,000 miles with the tuning box fitted. However I never tow and am fairly sympathetic with engine and transmission.