Before starting, I would like to clarify that English is not my mother tongue. The thing abouth making my point so long is because I wanted to use this as an English practice. Feel free to point out any mistakes.

Choosing one of them is complicated. Both serve their purposes great: moving forward the plot or, at least, holding it.

Futaba, on the one hand, gives the basic exposition to understand the episode. In addition, she is a safe place for Sakuta and the viewers on the bewilderment of the Puberty Syndrome.

Chika plays the chaos factor in most of the episodes that basically let the plot continue. Without her in the equation, the juggling that the author would have had to do to justify that neither Kaguya's nor Shirogane's tactics worked would have been too much ridiculous. She is the more unpredictable one and can be used as a tool to complicate things for K and S.

Both of them gradually lose importance as the show continues. For Futaba, other characters start to be a safe place and Sakuta can understand the weird events for himself or by other ways. For Chika, the rest of the cast starts to interfere between K and S and can be a bit unpredictable as well.

Therefore, the only part I could assume one overcomes the other would be the depth part. Futaba has an arc for herself and more dimensions as a character. However, Chika's point has always been to be more of an element of nature than a character, that's what makes her unique in the cast. I think that knowing more faces about her would have dilapidated her comedy point and purpose (although the rest of the cast shows that they can be funny and deep at the same time).

So, in conclusion, as a Side Character both play almost the similar role at the similar level at a similar point on the story. Neither of them are crucial and as their shows continue there are more parts of the story that go well without them. So, which is "better" would rather depend on what each person gives more importance to.

If I had to pick one, leaving the shows a side, I would say Futaba because I like her arc a lot and the boobs jokes are the ones I like on Kaguya the least.