Both are American companies. Whatsapp is Meta (Facebook), and iMessage (actually now Messages) is Apple. If you care about security, the question is which is more likely to comply with the US government's requests for information: Meta or Apple. If you don't, the question is price point, ignoring the question of which is more likely to make your data marketable. That is likely Meta, so we could rephrase this as, "Do you care to pay more for Apple's privacy, or is it okay for Meta to sell your messages to anyone willing to pay?"