I voted MK. We were there in early October last year, when they were first changing to Wishes, and IIRC, it is set up to surround the park, so you hear/see fireworks all around you. If so, that effect would be lost in a 7SL cruise.
We saw Wishes from Ohanas, Contemporary and right in front of the castle. To me there is nothing like being right there, watching Tink fly overhead and hearing all the ooooh's and aaaaaahh's, seeing the looks on my childrens faces. Truly Magical!!
Although if I wanted romance, then a cruise would be ideal!!
Even though it can be crazy leaving MK after the fireworks, I think the experience of just being on Main Street is well worth it. Walt wanted it to be that way, and stopping at the shops (AND buying ice cream ) brings a finality to the day and seals the memory in my mind.