I come from the old school where the rule is the more solid the better. I prefer 5/8" because of the large area and flex in the ceiling that you don't have as bad in the walls. Screw it at 8" spacing and tape it like Jay said. Houses are still being built in my area with 5/8".
I see 3/8" drywall used in mobile homes. Hit the wall and you can tell the difference.
As an example, when I grew up, we sheeted roofs with 3/4" cdx and now everyone uses 7/16" osb. I don't know if 3/4" was overkill, if 7/16" is structurally sound, or if contractors just got caught up in a long term bidding war while taking shortcuts. I don't know but then I also worked on houses when 2x4's didn't have a half inch shaved off of each side.
You have the final say so do what you want. You have to live with it and possibly sell it later. The price difference is not that much. Talk to the contractors, pro drywallers, and lumber yards/home centers in your area to see what is being sold and installed. You will probably get the same range of opinions. Good luck.