The lifetime subscription usually goes on sale for $200 (instead of the normal $300) towards the end of the year. Additionally, if you subscribe to annual, you get any remaining portion taken off the price of the upgrade to lifetime (so not the whole $89, but the amount unused). While there’s no guarantee the sale will happen this year, it has happened every year for the last several years, so it’s very likely.
Based on this, your best option is to get the annual subscription now, and then if you’re still consistently using WaniKani, upgrade to lifetime when the sale is available. This will save you money compared to buying lifetime now, and it will give you five more months to evaluate WaniKani before committing to lifetime.