I mostly message over WhatsApp but iMessage has a lot more features, and especially fun (e.g. multiplayer games directly in the messaging program) and useful (e.g. very slick location sharing) ones.
I use both, and I find WhatsApp much more fun, so maybe this is just a matter of habit/convenience.
iMessage is regularly glitching out for me, doesn't work in the browser, has a more confusing sent/received/read indicator system than WhatsApp in group chats (not that I particularly care about the "read" part, but knowing who actually had signal and received the message is useful when traveling internationally), and most importantly isn't available with all of my contacts.
WhatsApp location sharing works just as well in my experience.
