From what I've seen, it seems that one website offers the experience for people who want to get into the business of comics, and others who want to get into the social work of comics.
LINE is great if you think you're good enough to start making money off the bat, and want to reach the top. It shines when it comes to building a business for yourself, they do a great job of promoting creators and their series, buuut they're not too great when it comes to the actual community.
Tapastic is great if you just want to get your toes wet in the world of webcomics, with a friendly community that won't scare you off, and then maybe eventually look at monetary options in the future (Support program, ad revenue, Premium, etc.). However, downside to Tapastic (for me at least) is that I find that if you want any sort of promotion, you have to track down the staff yourself and you can never really rely on getting a good word in from them on their own. You have to find them, go through the whole process of making banners or whatever it is you're doing to promote yourself, etc. and the artists that DO get "automatically" featured are the SAME PEOPLE ALL THE TIME lol (ex. feyuca, min, people considered the "big artists" who basically make up half the advertising or marketeering for Tapastic). I have nothing against these artists whatsoever, hell, I love Feyuca and Min and Dave and all those peeps, but I find it's the same artists getting featured over and over again and when people ask why they can't seem to catch a break and Tapastic is like "well maybe you need to try different tactics" and they try and "optimize" their front page for more exposure for varied artists, I'm kinda like "UH MAYBE WE CAN STOP TALKING ABOUT THE SAME PEOPLE AND COMICS ALL THE TIME I WANNA READ SOMETHING NEW UGH". I dunno about you, but I've already gotten bored of Two Cent Slice and Randomphilia (when it used to be here and prominently featured) and Fail By Error is starting to get stale for me too, mostly because these comics are FREAKING EVERYWHERE LOL. On the front page, on the About pages, ERRYWHERE. I dunno, maybe I'm the only one noticing this?
That last point is just from my own experience though, I'm sure other people's experiences vary, with Tapastic AND with LINE Webtoon.