driving time to marne-la-vallee from either dieppe or calais is about the same, just over 3 hours; if you live near newhaven it would seem to be a no-brainer - although there are only 3 sailings a day, and they are timed so that you'd either get to paris in the early hours, or the height of the evening rush hour. calais might be a better bet if you need to be more flexible, notwithstanding having to drive to dover.
We have been to Eurodisney three times and have stayed at Davy Crockett - it's nice although you have to drive to the actual park itself. When we stayed there were some 'characters' about so even though you are not on site, you're still Disneyfied. It's ideal if you have little ones and need to do bottles, etc. I wouldn't stay there again as our children are older now and we can cope with rooms without kettles and a kitchen area.