Watch the originals first. That way, the original intents will be preserved.
As was previously stated, there are plot twists that would be ruined by watching the prequels first, and it was meant for you to see how it all comes together in Episode III.
I suppose you could watch the prequels first, if all that mattered was watching them in chronological order.
Here's an idea:
Watch them in this order:
4-5-6-1-2-3-4-5-6
That way you get to see them in the correct order, while still being able to see chronology after you finish.