Difficulty is now 300% higher than it was back then. TH/s machines should be arriving any day now which will make it even harder.
You just answered yourself. Invest in TH miners instead of older miners, because they are going to make all the money.
I agree that they will make the most money but there is no guarantee of a profit.
In fact, I fully expect > 30% difficulty increases to be the norm rather than the exception once these machines start shipping in volume.