As a healer, which character is better? Or, which should I spend exp materials on first?
Bennet and it's not really close. Bennet is a support and healer in one. While healing he can greatly boost your attack damage. Though he does require C1 so I would take Noelle over a C0 Bennett but that's it
Noelle can be a good consistent healer if you have her at constellation 1. Otherwise, I recommend bennett. Bennett can only heal from his elemental burst, but it heals pretty fast and can easily be equipped with proper artifacts for strong heals.
Be sure to use him on teams with another Pyro character, or give him energy recharge artifact/weapon for fast and consistent burst uptime.