img credit
Well from my experience I'd say definitely Anki because of:
customization and formatting options for cards (colors, italics, bold, underlining, images, tables, math formulae... you can get some add-ons or tweak the code if you know how to, i don't sadly lol)
organization of cards through decks, tags and card state (you can mark important cards, flag them with colors, suspend the ones you don't wanna see, change their deck, reschedule the review date...)
much easier to add and browse cards (this is my biggest gripe with Quizlet, how am i supposed to browse through a deck of 500 cards if they're all just lined up in an eternal list?)
can be used offline (though it can be synced across devices when you're connected to the internet)
there are probably other reasons that i haven't mentioned. that said, Quizlet does the job perfectly if you're not as picky as me. i think that's actually the downside to Anki, it's way too complex to learn so it may not be helpful if you aren't using it correctly. in my opinion if you're satisfied with Quizlet, you can keep using it. (haha i have a strong opinion on this topic and long-running frustration with Quizlet :D)
