Basically a 50-50. But I'm going to roll with Natsu. I've tried coming up with arguments for Jellal to win, but I think they're weaker than the arguments I can form for Natsu.
Natsu's offence slightly edges Jellal's as a whole imo. FDKM is probably the strongest offence in this tournament. One attack is enough to rival Grand Chariot. But as we've seen, Natsu can stack a few large Fire Dragon slayer spells right before a FDK spell. In a sense, Natsu's powerful attacks are a little more combat effective. For Jellal to oneshot Neinhart, he needed a little time. Natsu could enter FDKM fairly quickly, all he really needed to do was catch Jacob.
They both have similar physicals, on feats anyway. In my opinion, Natsu will be the one to generate better physical feats in the long run. He might actually have slightly less durability, but his endurance will be on the top tier like Erza/Laxus. Natsu has always been able to endure damage from powerful foes, even when he wasn't necessarily a top tier. Now he is, so I'm very confident his physicals would be top. Jellal's would be slightly below overall, but for this fight I'm happy to equalise them.
Speed's not a factor for me. Well for 2 reasons. 1) Both have the AoE to hit one another regardless of speed. And 2) Dragon Force. Even if Meteor is faster than base Natsu, DF will blitz Meteor. I'm not wanting to use Dragon Force at all, because if I did I see this ending with low-ish difficulty. I'd only use DF as a counter for "Meteor is going to blitz Natsu he won't be able to hit Jellal". So I think Natsu will be outdone by speed in this fight unless he uses DF, but even in base it's not as if Natsu will be blitzed all the time either, he's not slow.
Jellal's smarter, but in fighting they're probably equally experienced. Strategy probably doesn't come into play much, Jellal would have a slight edge here. Natsu fights intelligently imo.
Speculation - Jellal has Sema and Altaris to come. These are very powerful spells. Natsu has a FDKM secret art, and Dragon Force. In terms of speculation, Natsu would have the greater trump card. So if it came done to whose likely the stronger person? Then that would be Natsu. But for this fight I'm trying to avoid DF as much as possible.