Definitely the Swiss family Treehouse! Hands down. To spend a night in the treehouse would be amazing. Covered living area's complete with running water and great view of the park. Oh yeah! That would be the bomb! My kids agree with me.
My hubby and I have always made jokes about being alone on Tom Sawyer's Island but other than that I would definitely take the castle suite. Speaking of which, how does that work? Do they come escort you out of the park in the morning so you don't get an extra day? What about those people that start their day at 5 am.... or better yet... how well is it sound proofed for those people who sleep in until 10am? Haha!
On the Streets in Magic Kingdom. I would take all the time needed to appreciate the detail with no interruption or crowds to disturb me, and explore it all. (although I do think it might be fun to sleep in the Swiss Family Robinson Tree House.)