The difference is emotion and will.  

Procrastination is an emotional decision.  

Put it off because of fear, tiredness, or the unknown — that’s procrastination.  

But some things are put off because the time isn’t right.  

Because there’s something better you should be doing.  

Because I choose to not do this thing right now.   

That’s the choice of postponement.  

We’re always better off to make decisions as an act of the will.

Otherwise, the decisions are made for us.