Both kbin and lemmy utilize parts of the activitypub protocol - a generic way for different social media sites to talk to one another - to make a reddit-like functionality. This means that regardless of whether you are on a server which uses lemmy or kbin, they can access and use each other. The only real difference for users is going to be the UI and that kbin has also used activitypub to give its users some dedicated mircoblogging capabilities (think mastodon). My advice is: if you are only interested in a reddit-like experience then use the one with the UI and community you perfer, but if you want an all-in-one account (and are okay with the added complexity that comes with) kbin is closer to what you want.