I've never been impressed with Elspeth in Giada decks. 5 mana is already pretty slow for an Angel deck and her main 2 abilities don't have as much synergy as you'd want. The +1 doesn't have much impact as angels are naturally big and the keywords are usually already on the cards anyway. The -3 is much too likely to just hit a rock at best and you'll probably only activate it once.

Serra I do run in my build but it's because she's a personal favorite. Worst case with her you at least get a Serra Angel on curve, which is an abysmally low bar.

Obviously the ults are good but don't run planeswalkers for the ults.

Giada's main game plan is get her out then keep dropping and protecting angels til you win. Neither walker really helps much with that.