We will be there too (the 30th is our little ones 6th bday)...
mulanrouge - This might be good to do on the 30th for you. My thought for this is you have to pay extra anyway. Therefore using this on a day that you wouldn't want to use a normal park day might be a plus. - - - Also then you will be less tempted to run into a park, using a park ticket. Instead you can relax, check in, and know you have a bit of time to get ready. But how your family copes with the travel is the defining rule.
As for us, we plan on MK on the 30th (where else for a princess's birthday), but we are planning to do MNSSHP probably during the week before (maybe Thur).
Can't Wait!!!!