Decide by the page’s role in the funnel, and when the two goals genuinely pull against each other, split into two pages rather than compromise one. A top-of-funnel page should prioritize ranking, because its job is to earn the visit from a broad informational search, with conversion kept soft (a gentle next step, not a hard pitch). A bottom-of-funnel page should prioritize conversion, because the visitor arriving on it is already qualified and close to acting, so squeezing more ranking out of it matters less than turning that intent into action. There is no blanket “always rank first” or “always convert first,” the answer follows the page’s position in the journey.
The mechanism is what each kind of visitor needs. Someone searching an early, informational query wants an answer, and a page that ranks for that query has to satisfy it to hold the position, so loading it with conversion pressure both weakens the ranking and pushes too hard on a visitor who is not ready. Someone landing on a comparison, pricing, or product page has already moved down the funnel, so the page’s value is in converting that readiness, and over-optimizing it for a head term it was never going to own trades the conversion you can get for a ranking you probably cannot.
The split-into-two option resolves most cases where one page seems forced to do both. Instead of one page half-ranking and half-converting, create an informational page built to rank and earn the visit, and a focused conversion page built to close, then link the first to the second. Each page does one job well, the funnel moves the visitor between them, and you stop asking a single page to win a tradeoff it does not have to take.
For your next page, name its funnel role before you optimize it. If it is top-of-funnel, build it to rank and convert softly. If it is bottom-of-funnel, build it to convert and let the ranking be secondary. And if the page is honestly trying to be both at once, split it into two and let each one do what it is for.