As migeru says, the main question is how many, and which, member states back which entity. Neither has much power without member state backing.
Though the high representative already exists, the position will be more cristallised under the new treaty, and the president is an entirely new position.
How these positions evolve typically depends upon a number of policy fights -- that is, the institutions and their powers evolve as a set of commonly accepted rules of 'how things are done' emerges.
The rules governing the high representative and the council president are vague enough for that, from what I've read in the Lisbon treaty -- though I did not read everything. But it's also something you see throughout the history of the EU and international institutions in general.
For these reasons, the question which person will be the first in the office of president will be quite important.
So, now you still don't know the anwer to your question, and you should have some new questions :-)
The intent of the new treaty is to clear this mess by making the HR paramount. The letter of the new treaty has this bit about the President can represent the council internationally without prejudice of the HR's attributions. So letter and intent are clear. The HR also has to report to the Parliament every 6 months, not so the Council President.
This makes it clear why it is such a problem when Blair says "I'll be president if you give me foreign policy powers". We have met the enemy, and he is us — Pogo
