If your outbound is definitely going to happen it probably doesn't matter.

Buying two one-ways is good if you only need to change one leg as you can change the outbound withot triggering a re-price of the whole itinerary. But if the outbound is definite then you're only looking at maybe changing the return. At that point the change fee is what it is and you have to buy up to whatever fare the going rate is.

I wouldn't expect that day of travel there will be too many discount fares available, though I don't know that market specifically.