Memory vs Storage

storage.sol

pragma solidity ^0.4.19;

contract TestStorage {
    struct St {
        uint256  amountPaid;
        uint256  amountOut;
        bool registered;
    }
    mapping(address => St) store;
    
        
    function addMemFunds() public payable {
      St memory myStuff = store[msg.sender];
      myStuff.amountPaid += msg.value; // and the other members
      store[msg.sender] = myStuff;
    }
    
    function addMemFail() public payable {
      St memory myStuff = store[msg.sender];
      myStuff.amountPaid += msg.value; // and the other members
    }

    
    function addStorFunds() public payable {
      St storage myStuff = store[msg.sender];
      myStuff.amountPaid += msg.value;
    }
    
    function mine() public view returns (uint256 toPay, uint256 paid) {
        St memory thisSt = store[msg.sender];
        toPay = thisSt.amountPaid;
        paid = thisSt.amountOut;
    }
}