I would go with the northern route. I'm not sure how much of an issue Brussels will be, but Cologne and Frankfurt should be just fine. You'll probably hit Cologne somewhere between 10 and 11, which put you at Frankfurt just around 1. Traffic in these areas is only a major issue during the afternoon rush hours.

The A8 motorway between Karlsruhe and Munich is a mess at the best of times, a nightmare at the worst of times, I would try very hard to avoid it if you possibly can.