If one can stay unemotional, then a mechanical system should be a better option for the average trader. There are traders who consistently get over 175% returns following a mechanical trading system. Not that, I have already designed one and reaping the benefits. But I am working on it.
Since you asked for an opinion, my intention in adding to your thread is purely based on what I believe in, myself.
Mechanical system traders have to follow a preconceived plan without any changes or variations, control/minimize losses and take profits. The rules could be few but are constant. Advantages are that you can trade more markets compared to a discretionary trader.
A discretionary system trader on the other hand can vary his position size, rules as per the current news or events, meaning they can make better use of market information. Personally, I feel that a discretionary trader is more advanced but like I said in the beginning if a person can cut off all emotions and be disciplined to the dot while trading your plan, a mechanical trading system should be the best bet for the average Joe.
Finally, it all boils down to each trader's attitude. Guess, we all pay the tuition fees until we learn to improvise, keep learning and fine tune ourselves with whatever trading plan devised. It had to be exactly what we believe in, very personal.