How about his:
Remember the two variables:
Bitcoin Price and
Bitcoin Difficulty, buying a contract you counting on the bitcoin price will rise the same or more as the bitcoin mining difficulty.
This Difficulty rise can overtake the bitcoin-price growth overtime.
Is this more clear?
If you decide between mining and holding, you need to calculate in Bitcoins, not dollars. So if you invest 1 Bitcoin, days-to-ROI is when you recover this 1 Bitcoin back. The price is only relevant when you calculate maintenance costs.