The short answer: Google Ads captures demand, Facebook Ads creates it. Which one you start with depends on whether people are already searching for what you sell.
Google Ads puts you in front of someone with active intent — they typed your category, your problem or your competitor's name. Conversion rates are typically higher and data comes quickly, which makes it ideal for validating an offer and finding the keywords that pay.
Facebook Ads builds demand by putting your message in front of cold audiences who were not searching at all. It is better for products people discover, impulse purchases and building brand awareness. It needs more patience, creative testing and audience research.
For a B2B service or high-consideration purchase with real search volume, start with Google. For a product with strong visual appeal or a new category people do not search for yet, start with Facebook. Most clients end up running both — search captures intent, social feeds the top of the funnel.
Whatever you choose, tracking comes first. Conversion tracking and offline conversion data are what turn either platform from an expense into a profit center.