might try one just to see whatever happens
Personally, I'd just pull on the current promo banner for Bennett and Ning, and then stop there. Maybe try to get a copy of one or both, because their first constellations (unlocked with a copy of the same character) are both pretty good. After that, continue to save up again. Unless you REALLY want a non-promo five star on a promo banner, such as the current one, it's probably a good idea to skip all of them. If you can end up with the same character from "losing" the 50/50 on the promo banner, you probably wouldn't want to waste winning the 50/50 on that character.
Example possibility: Spend 78 wishes on this banner, get Keqing as the featured character, and get Keqing again when you're trying for a promo character on the next banner. That wouldn't be good.
OR
Spend 78 wishes on the next banner, get Keqing at random, and get a guarantee of the featured promo character the next time you get a five star.
It can take months to save up for a five star, so you generally don't want to end up with copies of the same five star character.
And yeah, I mean do all this after you've used up the beginner banner wishes.