As others have said, I have to recommend using all three. Feel free to mirror away! If you are offered a contract with one of them, you are free to take your comics down from the other site(s). You can notify your readers so they can continue to follow you and keep up with the comic they have grown to love. I've seen comics who moved to one or another for this reason have an announcement page thanking everyone and giving them the new information, so it can be done!!
Everyone's pros and cons above sum up everything pretty perfectly, and I pretty much agree with all of them. I'll just add my personal experiences with mirroring on all three sites.
Smackjeeves: Started on here. (Actually I started on Livejournal, HA!)
Pros
+ Like others, I love the customization.
+ Nostalgia.
Cons
- I have been spoiled by multi-page uploads and I don't like single page anymore.
- Traffic. I know for sure one guy has seen my comic. He seemed like a peach! But...
Tapas: My first serious attempt to upload all my comics in one place.
Pros
+ Forums and contests encouraged me to really dig deep and try a lot of new stuff. Even a novel!
+ Forums, period.
+ After the multi-page uploads I just can't go back.
+ I can have my own ad at the bottom of my comic (ko-fi, patreon, whatever) and I can get tips and ad revenue.
Cons
- Boy do you have to fight tooth and nail for exposure unless you get featured. Wow!
- And heaven help you if you are not romance or slice of life.
- For some reason my friends are left baffled by trying to read anything on this site, comics or novels. I don't get it, but it's really beyond their ken for some reason. I've heard this from multiple people.
Line Webtoon: Just now started.
Pros
+ For some reason I really like the interface. I guess it's more like the "MangaFox" etc sites I'm used to.
+ Seems faster to get into to as a creator.
+ I see more genres featured.
+ I found it pretty easy and quick to convert from Tapas format (bigger) to Webtoons format (smaller). I just chopped up the pages basically.
Cons
- Having a buffer is complicated. There doesn't seem to be an easy way to get to/see your series editing links unless you just plain save them.
- I don't even have a creator profile besides an impersonal "search Sandpaperdaisy" link that shows my comics.
- I don't have any way to track comments, so I have to run through all of my comics manually and respond to any comments.
- I don't have a way to put my own ad at the bottom of the comic. (It does automatically show my patreon though!)
Lastly, I have enjoyed starting each site at different times. It gave me the opportunity to take everything I learned on Tapas and refine it for Webtoons. I'm experimenting with compiling several of my one-shot Tapas comics into compilations for Webtoons, for example. The different platforms' strengths and weaknesses lend themselves to presenting the content in different ways, and I feel like the more you get to experiment with how you offer your comics, the better.
Because ideally, you will eventually find what works. And/or what you like best. But you can't find out if you don't try different stuff!
(And anytime you have the opportunity to get more eyes on your work, for free under a non-exclusive platform, you really should take it in my opinion.)