I honestly prefer how things were paced in the one-shot, but I guess the first chapter had to rush to get everything down, in typical Weekly Shōnen Jump fashion.
The first chapter is basically a 1:1 remake of the one-shot, with some minor differences. The first difference is that the duo's relationship in the manga definitely feels more likely to become romantic in the future than in the one-shot. Of course, they still feel like siblings, but this could change. I wouldn't welcome that, unless it's executed extremely well.
The second difference is Asuka. It's not very noticeable, but she is more blushy and slightly less of a doofus. I honestly slightly prefer the one-shot version of her so far, but I'm sure she will get more characterization in the serialization so it's a complete non-issue.
I thought the one-shot was solid. It wasn't anything groundbreaking, but it was a serviceable premise with serviceable characters and a serviceable level of fun and wholesomeness. I remember wondering how this manga would continue being about an ex-Hero Girl and ex-Demon Lord, since the entire point of the series is that they want to grow out of these roles into a more normal life. Does the series has any legs to go places? After reading the first chapter, I still have this question, which makes me sort of worried; a monster-of-the-week format would be a bit of a cop-out. I'm sure the mangaka has something cooked up, though.
Overall, this is a solid start to a solid-looking series. It will be fun, especially since we don't have many other series like it in Shōnen Jump; I'll probably be reading it every week just because why not. Hopefully it becomes something more though.