I can’t tell you how the rewards program works, but I can tell you about my experience with Inkitt.
I hated it. I can’t express that enough. First off, writing on Inkitt is a nightmare. It’s a separate writing app from the reading app and it’s so glitchy. Chapters won’t always save, the order randomly changes, bold/italics/etc. never stick, etc. Their support is nice enough, but the issues are never solved.
In the actual app, there is very little social interaction. You can’t comment on paragraphs like you can here, you can comment on full chapters but most people don’t, and you can write on walls but in my experience, most people don’t. There is no PM system.
The layout and look of it is boring. Their selection is small so it’s easy to get noticed, but the app as a whole is pretty small so how fast you get readers will basically even out with Wattpad.
I’ve also heard really sketchy things about their program but I can’t speak to that. I’ve heard they get the rights for the story and they can publish your story and weird stuff like that, but again, I don’t know all of that for sure. Just what I’ve heard.
One of my friends here is a pretty popular writer and he asked me about it as well, I told him everything I knew and heard about it. He decided against it.