I don't think you mention when you are travelling but bear in mind both of the ferry crossings (unless you take the Eurostar, of course) can be very badly affected by weather. Neither looks very daunting on a map but both the Irish Sea and English Channel suffer from pretty fierce weather during the winter (and at other times, though less regularly).

Passengers were stuck on the ferries at sea for up to eight hours last week because the Port of Dover was closed... they've been having storm force 10 & 11 gales throughout December and into January.

I would agree, by the way, about the Harwich to Hoek crossing - very comfortable cabins (assuming decent weather), good food and a generally pleasant way to spend the night.