WEBTOON originals are free-to-read, while Tapas Originals are not. That's the biggest difference IMO, as it impacts your potential readership - and a smaller readership will negatively impact the potential Patreon revenue you can pick up from any bonus content adjacent to your comic.
Beyond that, I believe both sites have you contracted for a set amount of panels per episode, on a weekly release schedule. Webtoon pay you for each episode you produce for them, then pay you extra on top based on the amount of pageviews/ad-rev generated, and the amount of fast-pass episodes purchased. I don't know if Tapas pays you per-episode, or if you're only paid a percentage of the coin and ad revenue that episode generates. (If it's the latter, that's not a very good deal. An Originals schedule is a full-time job, and it needs to pay accordingly.)
I know there are a number of WEBTOON creators who signed with WEBTOON early on though, who got a really raw deal.
Excessive panel counts, no hiatus breaks written into the contract, not enough pay to accommodate the assistants you need to hire in order to meet those crazy panel counts... that sort of thing. This is more common with older series, as newer creators signing with them have heeded the cautionary tales and been firmer in contract negotiation. (ALWAYS NEGOTIATE.) A good example of a comic which seems to have a more reasonable panel requirement per-episode is 'Of Swamp And Sea', which typically runs between 40 to 55 panels per episode - something I'd consider healthy and realistic on a 6-month-on, 6-month-hiatus schedule. (It's also excellent, currently one of my favourites.)
I know there are people on this forum who have actually worked on WEBTOON Original series in the past, who can likely shed more light. Not sure about Tapas Originals, but if there are, I'd be curious to see them weigh in with their experiences too.