I find that the biggest growth is when I get promoted by either site. The caveat is that your art and story has to be at a certain quality. I find audienc by being active on either of these platforms social media and activities and posting regularly on their discords, reddit, forums etc. that way, the staff will more likely to come across your work. It be that you're just more active in the Webtoon community than Tapas?
That said, I noticed that I got way more subs when promoted on webtoon (~450) than tapas (~250), probably because Webtoon has a bigger audience. berate obfuscation by Tapas.
Tapa's notification, user account, and stat system makes it a whole lot easier to engage with your following compare to webtoon though. For example, if someone sub to my comic, I can easily go on their page to see what other comics theyre subbed to and learn what kind of audience I have.