I like them for different reasons. Mastodon is somewhere I check on my phone for updates on things I care about, random posts, memes, or interesting topics that make it into my feed.
Lemmy on the other hand is somewhere I actually hang out. Sure, there are the memes, but there's more room for in depth discussion here, and I can spend hours here while I usually browse mastodon in minutes. In a way they are both how Twitter and Reddit used to be before they got massively popular and I find it refreshing.
In the end, I went with kbin because it combines features and integration of the two platforms while having stuff unique to it. I definitely recommend it if you like both platforms.