Which channel crossing you use depends on where you are in the UK and where you are going in France.
We travel from the Midlands to central France and have done long and short crossings plus the tunnel.
We much prefer Portsmouth - Caen with Britany Ferrys. It's easier to get to Portsmouth for us, We have to get round the m25 to get to Dover. The journey is longer in France as well. We like the longer ferry crossing, we can go on the overnight crossing and get to the port in time even after being at work during the day. We have a meal on the boat, a good night's sleep and off the boat and away in the morning. It is more expensive to do the overnight crossing (you have to pay for the cabin, wouldn't consider overnight without one) and if you travel on a day crossing there are children's entertainers and play areas as well as being cheaper. There is space for kids to run around, it makes a good break in the journey.
We have also used the cheaper western channel crossings with transmanche/ DFDS / OF Lines (all part of same group) who go from Newhaven/Dieppe (cheapest option) or Portsmouth Le Harvre. There are more "budget airlines" type, not as good food or stuff for children, boats not as nice, and we find the crossing times really inconvenient. Also, overnight cabins are even more pricey than BFerries and book up fast.
On the other hand, book with Transmanche on newhaven dieppe and you get 20% discount if you are under 25, a student, disabled, over 60. The discount applies to the whole group if booked by the person entitled....
The shorter crossings seem cheaper, but you have to factor in the cost of the extra driving including the extra motorway tolls in France ( about €30 each way for us) plus longer for kids cooped up in the car! However, you can still have a meal on board and there is play stuff and space for children to let off steam.
Some people really like the tunnel. We used it after Xmas this year as I had left it too late to get a ferry crossing. Yes it is quick, the loading and unloading is very organised and you are straight off onto the auto route.
But we don't like it. It's boring, you can walk about round the car, but that's all. No cafe, no real rest for the driver. You have to factor in more rest stops when driving to make up for this.
We do this journey about 5 times a year, and have worked out the inclusive cost of just about every combination! We have a frequent traveller club membership for BFerries which reduces their prices by 30%, and with this they work out only a few pounds more than the short crossing as well as being the most comfortable and convenient. If you know someone who is a member, they can give you their code and you can get 10% off as their guest. Could pm this to you if you like.
As for rough weather,we have never been seasick even in a winter Gale! Our kids used to find it exciting and were disappointed when it was calm. Modern boats have good stabilizers, and the only time it has been really bad was in the winter.