In 2009 we ate at the CP the night we went to the MNSSHP. Kids, DH, and I all wore our costumes, and we had a great time. The characters were also in costume. Our ADR was for 5:50 and that was great. The festivities do not officially begin until 7 but when we got out of dinner we found Lilo and Stitch in costume taking photos right at the entrance to Adventureland.
I am sure you will have a great time no matter what, but I would recommend dinner. You can go back to the hotel, relax, and come back and enjoy the evening refreshed.