Whatsapp is good but not available for iPadNot at all.
I found iMessage a pain to use. With messages not sending or not receiving some. In the end I switched it off on my iPhone.
I use whatsapp every single day and have used it across Android, Blackberry and iOS. I've never had any trouble with it and don't have to use it exclusively with iOS users.
There are other messaging options out there so I think iMessage shouldn't be the only reason holding you back.