speed of the crossing is winner for me, and frequency of crossings means bit early/ late or change of plans isn't big issue.
This I like, if it was within 3 hours, it would be my preferred crossing..

No choice of start time in Europe on the ferry, it is what it is, 08:30hr The train is much more flexible, you can head to France for mid afternoon, then hoon a few hundred miles and hit the sack, ready for a start the next day..

windy