I've got both of these flashes with 5d II and have visited Disney a lot.
The 270ex II is really light and portable. You can keep it on your camera all the time as it's super small and not too bulky for getting on and off all the rides. It's a great little fill flash, and would do an o.k. job with a 2.8 lens. The high speed synch has to be enabled through an in camera menu so that's a little bit fiddly. Also, it blast short bursts of light in stead of infra red to achieve focus in the dark. It's annoying. That feature can be disabled in camera but then you have no focus assit in the dark.
The 430ex II is probably the sweet spot between size and functionality. It has more bounce options, stronger power to overcome the noon day sun, easier access to control flash features like high speed synch. If you've got little girls and are planning to do a lot of portrait with the princess characters, then this is your flash. it also has a proper IR beam to help you achieve focus in the dark.
If was going today I would probably bring te 270 ex II to Disney because I have lots of good Disney shots and Disney is more fun when you pack light. If it was my first time though, I'd bring the 430 II with a stophen so I did not miss any moments.
Lastly - if your not on the 5dc and shoot a lot of video - you may want to check out 320ex for the video light. I've not experience with this flash.
Hope this helps,