We've done the St Malo crossing several times, and it has always been a fun part of the holiday (rather than a slog of a journey to have to get I er and done with IYSWIM).
Outbound, we get a 4-berth outside cabin for the night. The dc thoroughly enjoy the evening show and often have a tour of the ship which includes meeting the Captain in the bridge. We are 5, so I end up sharing a bunk with a dc. Dh does the driving, so I want him to get as good a night as possible, but I still sleep well enough, as the bunks are not stingily narrow.
Homebound, we get the cheapest, inside 2-berth cabin, simply to have somewhere to leave our daybags and have the freedom to move around the boat knowing there's somewhere where we can all meet up and eat together. Or have a nap (when dc were that little).
We generally buy supper and breakfast on the ship outbound, prices are not excessive, but homebound we are always laden with delicious French picnic food.
The shorter routes either waste a daytime of our holiday or arrive at inconvenient times/locations. We never feel that way about the longer St Malo route. Some of the shorter routes mean we would have to leave very early on the last day of the holiday, but not with the St Malo route, so that's much more comfortable, too.
HTH.