Both are correct most of the time but "whom" is formal. In the words of CoGEL (1985 edition, § 6.35), whom can be avoided altogether in informal style.
When the pronoun follows a preposition and that it is a prepositional complement the form "whom" is more regularly used. Fot instance, you say normally
- "This is the person who you spoke to.",
but
- "This is the person to who you spoke."
is not correct (unacceptable in British English).
In the words of CoGEL (1985 edition, § 6.35), the reason is that there is a stylistic incompatibility between the preposition + relative pronoun construction (to whom), which is rather formal, and the use of who rather than whom as prepositional complement (who … to), which is informal.
However, your sentence is not of the sort discussed above; "who" is not the prepositional complement of "of", instead "whom to choose so they can grow" is. Consequently, "who" is a possibility, albeit the style will be informal.
- So many decisions of who to choose so they can grow
Formal
- So many decisions of whom to choose so they can grow