I have used all three sites, and I was published through one of Inkitt's contests as well as them accepting another series we wrote. Tapas is now the only site I publish on for many reasons and I have completely removed my writing from the other sites where possible.
I quit wattpad because of the toxic negativity that fills the site, and the near impossibility of actually garnering any attention if you have real, original wok. I had garnered about 2k followers over the three-ish years I held my last account (in about seven years of using wattpad, I deleted several accounts because I was inevitably hounded by some hellish reader who thought they could rewrite my books and often attempted to on their profile, long story). The site is extremely overpopulated, and the only stories that get attention are the ones that are carbon copies of the same thing (bad boy meets good girl, unfortunate girl comes up into fame/fortune/love, and tragedy porn) or fanfic of extremely popular subjects.
The only reason I would ever think of starting another account there is if I wanted to write fanfic, but even then I'd post on AO3 over wattpad because the community is much more pleasant there.
The community is really the biggest reason why I left. The constant 'MOAR PLZ' and people who thought I would change my story because they cussed at me through private message drove me away. I also saw somebody literally make a hate account for somebody who defended me once. Tl;dr don't do it.
I quit Inkitt because they are literally the biggest POS I've ever seen in my life. They advertised the hell out of themselves and that's why I got interested- I was part of the email spam chain that was broadcasting one of their contests. You get no attention outside of these contests, even if you've been previously published through them.
And let's talk about the publishing. I won the contest with a book I published with my cowrited, and the entire process has been a complete wreck. We published November of last year after one round of half-a'd editing, a cover we were relatively fond of, and hardly any advertising as far as I could tell despite their promised 6k advertising budget.
To be fair, we made #1 bestseller in the LGBT section for like... an hour.
Things quickly went downhill. We got a lot of bad reviews because of the awful editing- though we did get some good reviews too.
Things really went bust when we inquired if they wanted the rights to the prequel of the book they published- because they had first rights for anything related to the published book. They said yes... and nothing happened. Four months later we brought it up again and they said 'lol no'- which I was extremely pissed at, because even if we had to self pub it, they lost us a ton of momentum we would have had for that book from the publishing of the previous book. There were several reviewers who were asking for that prequel without knowing we had written it.
For some reason we were stupid enough after that to offer them a six/seven book series we have posted on wattpad which needed MAJOR editing (and honestly I never wanted to see published because it's not the best quality). They said they'd do a 'trial run' before deciding if they wanted to publish it. I figured they'd put it up on the site (we had already put up books 1-3). They went radio silent since then.
If I hadn't been searching up our pen name on Amazon I never would have found out they had published the books without telling us. No editing at all, straight from wattpad to Amazon. No advertising, we have maybe one review from a wattpad reader who didn't buy them. No choice in cover design (though I like what they did). Basically they violated the contract they made us sign several ways and I don't care enough to hire lawyers.
All the attempts we have made to get into contact with them have come to nothing.
Don't do it. Trust me. If you want to build a name for yourself as an author you're going to start it out with that company publishing unedited bull.
I stayed with Tapas because I really love it. Genuinely. I haven't found anything I really dislike enough for it to bother me. The community here is wonderful, both the forum and my readers who have been very nice and supportive through a currently very difficult time where my updating was spotty. I love the ability to make ad revenue- however small that it- and receive tips from readers who feel generous. Being able to schedule chapters ahead of time is a godsend. It's hard to get noticed around the premium novels, but I've garnered a decent following without really lifting a finger.
I'm really happy with this site. Still working on ways to advertise here since I've started to slow down in gaining new subscribers. But it's definitely better than the other two choices.
Just keep self-pubbing or traditional pubbing- even through indie pub companies, which are growing in popularity- in mind.