They're equally "secure" in that everything is E2E encrypted. Signal has self destruct messaging and hides metadata (which Apple doesn't collect anyway) which would be classified more as features than security.
People don't use it because they have to get everyone they want to connect with on it. Where iMessage is already there and ready to go for all iPhone users.
Not really a fair comparison. iMessage is only E2E if everyone enables that and doesn’t back their messages up to iCloud. How many people do you know (besides me) who don’t backup their phone to iCloud?
And if you go with WhatsApp, then you have them sharing your metadata and contacts.