Pre AW Kerry has
+1 HP, +6 ATK, +33 DEF.
post AW Kerry has
+44 HP, +195 ATK, +41 DEF, but slower attack speed. Emilia has 15% chance to block enemy attack.
Emilia's advantage over Kerry is in her skill and if you get her cost below Kerry's.
I do personally use my Emilia more because she is 2UP cheaper and her skill if she is deployed long enough to matter.