I would love to say that Fairy Tail is just a fodderverse full of useless characters however I feel similarly about One Piece, so I will try to answer seriously.
Let's take a look at their main abilities.
Ace is a logia user, therefore his entire body is made if fire. He is able to control fire at will, he also can absorb any fire shot at him and allow solid objects to pass through him.
Natsu rings in with similar abilities. He is the Dragon Slayer of Fire. He can pretty much control fire, but not to the extent of Ace. He has a few large scale attacks and has the ability to regain stamina by eating fire. Plus, after the time skip he is also able ti use electricity in conjunction with his flames.
Theoretically, Natsu could just easy Ace, but let's asume that that is impossible.
Also, theoretically, Ace can just absorb the fire Natsu shoots at him but as we have seen by Aces death st the hands of Sakazuki, anything hotter than Ace can burn him.
This pretty much brings this battle to a stalemate. Unless Natsu's fire is hotter than Ace, Ace is untouchable. Any fire Ace shoots at Natsu, can just be eaten.
So, what I think will win this battleis speed. I have observed that One Piece characters are faster than Fairy Tail characters save for the fastest Fairy Tail characters. This will turn into a physical battle when both characters realize fire and lightning are useless. I think Aceis faster and stronger and will win with mid to high difficulty.
I hate them both yet keep up with both series anyway. Don't ask me why.