I have a garden variety A6 2.8 in the USA.  I would have prefered an A4 but
the family prevailed and liked the larger size of the A6.  I'm used to
performance cars, so here are my thoughts.

The A6 is a fine car but doesn't really encourage spirited driving.  It will
go fast, and handles fine, but it doesn't scream "drive me hard!"  I would
prefer the lighter, more nimble A4 for that kind of thing.  The A6 does
everything well, I just don't consider it a 'drivers' car, at least not as
much as I would like it to be.  I like blasting around within reason, I
think the A4 would be more to my taste.

If space is an issue you won't go wrong with the A6.  As you noted the boot
will take everything you could possibly want to take with you, and probably
have room for your neighbors kids too!  It's like some kind of optical
illusion, how could that much space possibly fit under that skin?

Either one is fine, if it were up to me I'd go A4.  I'd expect reliability
to be about equal, unless there are issues I'm not aware of.

Alan

I have an A6 2.4SE and my experience of it is:

Check the front suspension for wear, it is one of their weak points
and parts are not that cheap because they are mostly made of light
alloy. Listen for creaks and rumblings when driving it and check for
eneven tyre wear.

The petrol V6s  suffer from oil leaks usually caused by a blocked air
pump causing high crankcase pressure and that blows the rocker box
gaskets and camshaft seals. The pump is only £16 but the repair to the
oil seals can be expensive. The engine gives good performance for the
size of car and economy on a motorway run can be around 30mpg.

Cambelt - make sure thay it has been changed according to the book. On
the petrol V6 it is about £700 at an Audi dealer.

The handling is not that great, better suited to sedate driving, but a
good motoway cruiser and the seats are comfortable with good legroom
in the back. The wrong tyes can make it very noisy inside and the
right tyres make it a quiet car. There were Kumhos fitted when I
bought and they made a terrible noise; changed them to P6000s and it
went very quiet.

I was lent an A4 2.0FSI for two weeks about two years ago. Good
engine, bland car.