IMO it depends on what you want to lay emphasis on. You want the reader to know from the beginning that you are against or for an argument.
It also depends on what most readers believe in.
If most people believe this is right, but you want to tell the reader that in fact it is wrong, then you should start with positive points and then move towards negative points.
If most people believe this is wrong, but you want the reader to understand that this is right, then you should start with negative points and slowly move towards positive points.
Now with whichever you might be starting, you should keep it to a minimum, you want the reader to understand that these positive point are not enough or these negative points don't matter so much.
If you're agreeing with what most people believe in, then you should start with whatever they believe in - positive or negative and in the end talk a bit about the opposite.
Of course in the intro you should give a hint as to what you believe in.
My explanation might not be clear - explaining something this complicated by writing here is terribly difficult.
Remember everything written above is just my opinion.