Leo says that he uses a combination of Samsung's backup account and Google. Just by logging in, it should all transfer to the other phone. He wants to know specifically about his WhatsApp contacts, though. Leo says that WhatsApp should restore the contacts when he logs into it.
The chatroom says he can backup his contacts to an SD card via WhatsApp and even back them up to Dropbox. Here's a technote on how to do it. Leo also recommends having all of his contacts and settings on Google.
For apps, he should try AppBrain, which is a great third party app backup option.