I need help understanding the best implementation of function modifiers

Question

pragma solidity ^0.4.0;

contract SomeContract{

   address public owner;

    modifier ownerOnly{
        if(msg.sender == owner) _;  //would this use less gas than the modifier below?
    }

    modifier ownerOnly{
        if(msg.sender != owner) revert();  //is this modifier more secure?
        _;
    }

   function SomeContract(){
      owner = msg.sender;
   }
}

Answer 1

I created a simple contract and tested it in browser-solidity:

pragma solidity ^0.4.13;

contract TestContract {

    address owner;

    modifier ownerOnly1 {
        if(msg.sender == owner) _;  //would this use less gas than the modifier below?
    }

    modifier ownerOnly2 {
        if(msg.sender != owner) revert();  //is this modifier more secure?
        _;
    }

    function TestContract() {
        owner = msg.sender;
    }

    function foo()
    ownerOnly1    // or ownerOnly2
    returns (uint answer)
    {
        return 42;
    }
}

Here are the costs for calling foo() as owner:

For ownerOnly1: transaction cost 21570, execution cost 298.

For ownerOnly2: transaction cost 21573, execution cost 301.

However, when calling foo() from another account, ownerOnly2 causes an exception, but ownerOnly1 does not (returns 0 instead of 42). I suppose the ownerOnly2 effect is desirable.

Note that you can write this condition in a more readable way: require(msg.sender == owner) (see documentation on error handling).