Caster dilemma

Just focus on Tamamo for the meantime since she's better than Medea Lily, her skills has an Arts Boost, NP drain and Defense up for survivability. Her NP has skill cooldown decent heal and NP gauge fill so she's perfect for stall teams and servants appreciate the cooldown specially to those with maxed skills.
Medea Lily on the other hand is only good for healing but she's very good on that role. With a skill to charge her NP to make NP spams, you can heal your party with huge chunks of health every now and then.

Tamamo is the second best support servant in the game right now, and THE best arts support servant, even in JP. Medea lily and Illya are low tier. Max out Tamamo and put her on all for support servants, and you'll be rolling in friend points. That's how good she is.