In my opinion the Rouen route is definitely the way to go, the Paris route is too unpredictable.
The Rouen route seems to get easier every year as the roads improve and even though Rouen itself can be a bit of a bottle neck it rarely holds you up for long.
I always make sure that I carry a french road atlas with me, that way when somebody stuffs their car on a single carriageway section (as they often do), I can take to the country lanes for a few miles and divert around the holdup.
Cheers Stuey