They're statements, rather than idioms, and, apart from the lack of capital 'W's, they're both correct grammatically. The second one is something which you're more likely to say, though. 'What's mine is yours' means that I'm happy for you to consider my belongings as your own. It's a generous offer. 'What's yours is mine' is less friendly - if you say this, you're laying claim to my property. Nobody would be very happy if another person said this to them.

"what's mine is yours" is correct. "What's yours is mine" means the person is probably about to steal all your stuff, or something.

The above are correct, but very often we use them together. "What's mine is yours and what's yours is mine." It's something that like a couple would say about their stuff/money in a marriage. It just means that they share everything. But as I said, the above is correct about "What's mine is yours" is the one you would want to say to tell someone they are welcome to treat your home like it's their own.

The correct idiom is: Mi casa es su casa. (Spanish) (My home is your home.)