Many thanks for advise!
To be honest I am a bit torn apart as people from LH forum (I posted same question there) seem prefer A8 route... One guy travels from Brussels to Salzburg quite often and it seems he also prefers south route.
When did you travel A8 last time?

To be fare, people opinion split 50/50 and many people are as well unhappy with north route (roadworks on A3 and Koln-Franfurt stretch) as with south one...

I know there is one easy solution to take such journey overnight and avoid most of traffic (but it is a bit too much tiryng and I have already booked my cross channel ferry).

Ok, i seems I will have to take a pick as I will go along and will have to decide in Belgium/Namur about which direction I am heading