Hamayumi gives a 16% Normal Damage increase and a 12% Charged Attack Damage increase at Refinement 1, going up to 32% and 21% respectively at max Refinement.
Ayaka's power ceiling probably won't be powercrept anytime soon, but it's much easier for the devs to design content that counters her.
Bennett Bennett is a good alternative to Xiangliang, and for some, he's probably even better.
As a ranged team, they can deal damage at all times, and be healed, shielded, and buffed.
Get whichever you want they are both extremely good but I would say Ganyu is slightly more versatile but ayaka is more fun to play without c6.
Increased damage taken begins at 5% and increases by 5% every 3 seconds, up to a maximum of 25%.