That is an odd pair of alternatives to choose between as they are
different sized cars. Maybe you meant to choose between the allroad and
the A6 Avant Quatro as those are built on the same size of chassis and are
similar in size to the BMW 5xx and Mercedes E Series.
All I can say on the topic is that I like my allroad (which I chose
because I do need to regularly use it off-road), but I don't know anything
about possible risks of getting a used one.
What's best depends as much on the user as on the car. they're
different cars designed to suit different users. Do you have any
particular reason for wanting the Allroad? If not, why go for it
rather than a 'normal' car? Personally, I don't see the point of the
Allroad. But I'm sure that others think it's the bee's knees.
I recently (April) went from a BMW 323i (E36) to a three year old A4
3.0 quattro Sport... and don't regret the move at all. The Audi has
plenty of power for me and is far more comfortable to drive (the seats
are much better designed, as are the ergonomics). I'm not saying that
I find the Audi better or worse than the BMW overall, but it is a very
different car. For goodness' sake test drive the Audi before you buy
it. You might be surprised at how different it feels, and possibly not
like the difference.
Another poster has pointed out, quite reasonably, that the A6 is a
closer equivalent to your present 5-series than the A4. However,
although this looks correct on paper, I found that the change from a
3-series to a nominally equivalent A4 gave me what seemed a
significantly larger car. It's definitely more spacious in both the
cabin and the boot than the 3-series. (I assume that this is a
consequence of having a V6 instead of the longer straight 6 in front.)
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