To some degree they are just web frontends to the backend technology. kbin was done by this guy earnest which had a feel I just liked better but something happened with him and the fork mbin is the only real option now which is fine but earnest seemed to have a vision that did not come to complete fruition, I did not use lemmy much but when I compared the two I went with kbin. Was on mbin when kbin blew up but piefed came along and eventually I tried it. Its main thing is it has these topics where it nests communities. So I can browse the topic comics which has a bunch of communities where people post comics in it. Its handy. I really like some of the nice options like I can block on keywords and allow nsfw to show (I swear some people tag pictures with it because its showing to much ankle. ok that was an exaggeration but man peoples ideas of what is not safe I think is a super broad spectrum and im not really concerned about nsfw) in your feed without blurring and another than will sorta darken bot submissions a bit so you can tell when scrolling. Now all this is because its been super active with development so it can bit a bit wonky. when scrolling I see this thing where text overlaps and messes things up but ultimately its a minor nuissance and I can scroll up or down to get what I want and the other things are so useful plus I can see more is to come so seems to be the most likely to keep on getting better. Anyway that is why I cam where I currently am. Its gotten a spike in popularity too which caused it to be sorta offline for a bit a week or so back but it was pretty short and they got things going. I can tell you anyone used to earnest with kbin will think nothing of that (kbin was legendary with its downtime). So if you want more stability go with lemmy/mbin but if you want more features go with piefed. Keep in mind stability is mostly about instances so a particular lemmy or mbin could be more or less stable. Anyway I like features.