Setting aside the “is it legal/ethical” argument, you will not get the awesome level up emails with the Anki option and that’s just sad.
Wanikani is structured to teach you everything in a specific order that really helps. You have a set of radicals, a set of kanji, a set of vocab, kanji, vocab and back to radicals. The growth feels much more natural. It won’t do you any harm to work on an Anki deck. If you’re concerned about price, buy a one year subscription and wait to upgrade to lifetime come the sale around November - January.