It comes down to personal preference and what you’re looking for. Lemmy is going more true to Reddit in overall feel. Kbin is slightly different in that you can also view and post content to Mastodon. Otherwise you’re going to be able to see the same content so whichever works best for you.

The main reason I gravitated to kbin was that lemmy’s feed was bugged and would constantly refresh when you scrolled which didn’t exactly create the best user experience plus some issues signing in. But really both are great options! I’m subscribed to many lemmy.world communities.