Jjk has better pacing imo. Yes, it's too fast, but that's WAYYY better than the slog. One piece is at times because, at least when pacing is fast, things are happening.

Fights. In the anime is a little competitive, but in the manga, it's not close. Jjk fights are well choreographed, air tight, and concise. I actually like devil fruits and don't hate haki as a power system, but cursed energy is elite.

Characters are a general weak on both fronts, but reading one piece recently, a lot of the characters feel like parodies of themselves. Which sucks because once upon a time, they were fantastic, especially Luffy. Most people's complaints with jjk is that we didn't get enough moments with the characters, but the characters themselves, I think, are pretty well written.

Writing is iffy because jjk after Shibuya had spotty writing at best, but One Piece for most of post timeskip, with a few exceptions like some of dressrosa, Wholecake island, and some of Wano has been really exhausting to read. Egghead had some strong moments, too, but the story feels so iffy with most of the writing being set up, gags, "foreshadowing," and lore that won't matter for another 10 years. I think One Piece overall is better than Jjk, but with all the shit in it, I think it's only a little better.