It can matter. The investor's country of chargeability determines the Visa Bulletin wait, and the wait for a visa number is not subtracted from a child's age under the Child Status Protection Act. Choosing the spouse whose chargeability is current can protect a child close to 21.
Chargeability and the child's birthday are linked
Derivatives are the investor's spouse and unmarried children under 21. The Child Status Protection Act freezes a child's age by subtracting the time the petition was pending from the age at visa availability, but it does not subtract time spent waiting for a visa number in a backlogged category. Chargeability normally follows the investor's country of birth.
Hypothetical example: in a Sainte-Brigitte-de-Laval household, one spouse was born in Canada and the other in a country whose EB-5 category is backlogged in the Visa Bulletin; their child is 19. If the backlogged-chargeability spouse is the investor, the child may turn 21 before a visa number exists and lose derivative eligibility; if the Canada-born spouse is the investor, the category is likely current and the child's protected age holds. The choice affects nothing else about the family's rights, since all derivatives receive the same conditional residence, so it should be made on chargeability and on which spouse's funds and source evidence are cleanest.