With the lock in you guarantee profit regardless of getting the free bet, that's built into the spreadsheet's calculations. However, you get something like £5 on a £50 acca. The benefit over the 'normal' method is that you get approx 10% of your stake each time, whereas with the normal ones you make small losses until you hit a refund. So I think it depends on whether you like to guarantee small profits or take small losses and hop to hit lots of refunds. With both of these, each leg must be on at a separate time to lay off in turn.
The lay all method is different again. You usually make a profit off that, but not always. The benefit of it is not having to lay off each leg at a time, but it's more restrictive in what you can pick for your selections as the odds have to be quite tight to make it work.
If you're on PA I suggest having a good long read of all the methods and doing some dry runs with the spreadsheets before doing any with money.
