Should I get her E1 or E2, which has the best value?
E0 to E1 is a larger damage boost than going from E1 to E2. I'd personally save up for a Robin E1 on her rerun if you dont already have that. The 20% teamwide res pen is a huge dps boost especially against enemies with elemental resistance.
Her res pen is 24% when she has her ult up. It's pretty much ruan mei's ult (25%) already.
Oh you're right, I always thought it was 20% off the top of my head. But yeah that only makes her E1 even more valuable for Yunli and other fua teams.
Yes, her E1 is really strong.