Depends which school holidays you have in mind.
Shorter crossing give you less cost and the fun of driving down thro France - for us an enjoyable part of the holiday, and you may find the costs break even against the long ferry crossing.
Suggest you get CC Caravan Europe Volume 1 and have a good read, then do some internet homework on the ferry prices.
If, however you are thinking of half term then I guess the long haul crossing are best, as at least one will get you down to northern Spain in about 18 hours.
look at some of the other "first Time etc" posting on this forum and in the archives - there's a wealth of stuff there.