I’m not sure on the privacy side cause I didn’t really look into it, but for content in theory you should be able to pretty much see the same content on either site. In practice kbin is still very new and has some issues / is slow with federation. This means you currently will see more posts on lemmy than on kbin.
Personally, I like kbin’s design better and use it more (if you’re on mobile its mobile browser site looks great and can be used just fine until we get an app for it). If I run out of new stuff that interests me on kbin I switch to lemmy to catch all the stuff that didn’t show up on kbin.