You can't always go with the whales choice. As I recall, the whales backed a girl that didn't win.
Simply put: pick the girl you think is cutest/sexiest/best. It doesn't matter in the end. Your ranking has nothing to do with what princess you pick. You can still get the limited SR for getting 50 wins. And you can still get as many gacha tickets. Even if you pick the girl that comes in third, you get ONE card that's N. I didn't even like any of the cards last time, so I don't really care that mine became the SR.
Also, a whale can't really influence the outcome very much. You get 5 princess points whether you win 5/5 on a level 1 team or a level 5 team. This event is won by the people as a whole. The multipliers will keep it a relatively close race throughout.
I picked the girl that I liked best: the catgirl. At the time I picked her, she was in 2nd, but she's now winning.
At the time of this posting
1: catgirl
2: flute girl
3: axe girl
I expect it to change throughout the day and throughout the event.