The walls get 1/2".
Bathrooms get 1/2 but should be water resistant.
I only use 5/8 on the ceiling, but some of the new light weight is rated for up to 24" spans on the ceiling.
I'd rather not have to deal with call back for nail pops or sags that's why I use the 5/8.
The ceiling get done first. Got a lot to do then rent a drywall lift. A lift can really speed things up and make it a whole lot easyer.
Once the ceilings done, install the drywall on the walls sitting horizontal for less seams.
Intall the top piece first so it's tight to the ceiling. The bottom piece is kept 1/2" up off the floor. They sell a simple kicker to aid in lifting the bottom piece or you can just use 1/2 scraps.