When you have 20kg of camping gear in your panniers and you're riding up a 10% climb?

The correct combination is the one which gives you the lowest and tallest gears you need while keeping the most common gears you use on one chain ring. Strong riders can get away with using a 53/39 because they spend most of their time in the 53 and will only switch to the 39 when going up hill. If you find yourself normally using the 53 on the flat but needing to use the 39 when its a bit windy or there is slight rise then you may be better off with a 50/34 as it will let you stay on the big ring longer.

Really, the only way to know which is best is to examine what ratios you are using now and then figure what setup will work best for you.