Depends on which one you think is easier to craft.
Just for comparison:
Ankh Shield: 11 total ingredients (9 random drops, one crafted, one found in a chest), able to be completed in Hardmode before Mechanical bosses.
Celestial Shell: 4 total ingredients (all random drops), able to be completed after Golem.
Obviously you're going to want both though. Both are great on any build.