Let's see, Jason's come back from the grave as a sadistic arse-hole with no compunction against killing. He's been trained by one of the greatest fighters on the planet, then re-trained under Ra's al Ghul and Talia once he re-awoke.
Bucky, now that we know more of his past, was a well-trained army operative. He was the partner of one of the Marvel U's greatest fighters, plus was trained as a sniper and killed on numerous occasions. Once he was revived, he received extensive training from the Russians, not to mention his cybernetic arm.
I can't give you a tangible reason why, but I have this perception that Bucky is the more skilled fighter. He won't be surprised by Jason's ferocity and can handle his dirty tactics.
I think Bucky comes out of this one pretty beaten up, but eventually puts a bullet between Jason's eyes.
Both are incredibly skilled, but you have to bear in mind, Bucky's had leagues more experience in combat than Todd, dating back to WW2. Bucky was trained by various governmental factions, and was a deadly scout while everyone saw him as the wholesome sidekick. Todd has brutal methods of his own, but I think that Bucky's scrappy durability would prevail against Todd. Winter Soldier takes this.