I would vote for Sci Fi. We've eat at 50s PTCafe twice before. The first time (in 2005), the food and the service were excellent! The second time (2008) the food was good but the service was OK. We ate at Sci Fi in September and loved it!
You might want to make the reservations for a late lunch instead of dinner. The Halloween party officially begins at 7, but you can enter MK with your ticket as early as 4. You want to be there when it begins. We had loads of fun at it last year.