It depends on whether people already search for what you sell.
If they do, Google Ads usually pays back first. Shopping and Performance Max campaigns built on a clean Merchant Center feed, plus search on your highest-intent terms and remarketing to past visitors.
If they do not, or your product needs to be seen to be wanted, Meta ads come first. Video and static creative on Facebook and Instagram introduce the product, and Google then picks up the branded searches that follow.
Most Shopify stores end up running both. With Shift, education-led Meta ads introduced a brand-new product category to cold audiences, and intent-based Google campaigns captured the searches that awareness created.