The verdict
For a small B2B team, pick X Ads. It needs less production, targets by keyword and conversation, and has no minimum budget, so the test is cheap. It works best when you have content tied to launches, events or live topics.
For a direct-to-consumer or e-commerce brand that can make native-style video, pick TikTok Ads. The audience is younger, and commerce tools such as catalogue-based product ads suit shops.
If you sell a complex product to an older buyer, check LinkedIn Ads first. Whichever you test, set a fixed budget and a stop rule, and judge the result on cost per customer.