Mace, because in the book he is tied with Palpaltine. They are going to kill eachother in the fight if no one ifterfears. Later, when Yoda and Obi-wan are deciding who to fight, Obi-wan wants to fight Palpatine instead of Anakin but Yoda forbids it because it says he will NEVER be strong enough to challenge him. Maybe Yoda just thought that he would lose and was exagerating, but Mace tied with him. I think Mace came as almost as close as Yoda to beating him.
I agree Mace is a more devoted swordsman, but Obi-Wan has the resourcefulness, endurance and persaverence to win here. And if he did, it would be an extremely rough battle for him.
Also, Obi-Wan has been in more action during the clone wars, so you could argue that Obi-Wan is at least Mace's equal. Yes, Mace was a general too, but from my research, Obi-Wan was on the front line a lot more. thus he could reach the same level as Mace.
However, Vaapad is the X factor here, and that's why Mace would likely win. It attunes you to the entire force and that's why he matches Sidious. (Even though Sidious does let him win)