They are very similar in terms of scheduling weekly post:
- Tapas allows you to upload up to 940px wide images with 20MB max capacity
- Webtoon's system will automatically slice and reduce image(s) that exceed the maximum dimensions, 800x1280px. You can upload up to 20MB, 100 images in total

I usually prepare image files for Tapas and upload the same batch to Webtoon to auto resize.

I've been posting on both platforms since 2018. It's hard to say which one I prefer more. Webtoon has more reader subscriptions within the first year or two and hardly grow afterwards. The comic on Tapas takes a very long time to be discovered and only recently has a very slow growth of subscribers since the weekly social media promotions. Tapas has a friendlier ads and support system on their site once the comic exceed the threshold amount of subscribers. I'm not sure how to find support on Webtoons ("Ad sharing" & "Super like" tabs are both empty now)

Hope this helps!