If you aren’t sure you’re going to be using the lifetime membership, don’t do it, IMO.
I do monthly, because even though I really really really want to learn Japanese, there’s a hundred reasons I might quit, ranging from a lack of self-discipline, to outgrowing the site (perhaps I’ll find I have a better time learning kanji in the wild after a certain point? I don’t know.) to actual disruptive life events.
I try not to pay for something I don’t know I’ll use, so I’m happy to pay the crabigators 9 dollars a month. Should I ever end up paying more than 300 dollars, then wanikani has earned that money.
Now admittedly, the discount might change that calculation, but until then, I’m happy to wait.