Lol showing off. I really want to reel out the unconscious pic of Jellal after a weak Neinhart attack now
That's true. For moves like Sema, with a small cast time, I would consider these moves as finishers too. It's like saying Makarov will oneshot people with Fairy Law in a fight, most people will hit Makarov while he's casting and not stand there through CiS/PiS.
I agree that Jellal is probably going to adapt to account for DF's speed and maybe Jellal goes for AoE (you're throwing me off here with Gerard lol). The main factor will be that Natsu can now make this fight on equal or his terms rather than have meteor blitz and dictate the range and pace of the fight. If it comes down to CQC then Natsu will have an advantage, but maybe Jellal can endure it and get some licks of his own, and perhaps pull off powerful spells (though this goes both ways).
Physically neither have impressive feats so far, and this will be a factor in a fight with they both use high powered spells. I'd say Natsu would have better endurance, and their durability would be around the same. Jellal would have more MP imo.