I agree with you that Mastodon and Lemmy are quite different - the point of the fediverse is that you can communicate across them if you want. Here's our conversation showing up in Mastodon:

Lemmy is more like Reddit - principally short, text-based, Mastodon is more like twitter and there's a lot more graphical stuff there. Personally I find lemmy more intuitive - but I was never really into twitter, whereas I wasted a ton of time on reddit.

Interestingly in the screen shot above, the reply I made to the other poster about the NSFW instance that shows a graphic doesn't show up - I don't know why that is, but you can see our exchange shows just fine in Mastodon. In short, I prefer Lemmy but Mastodon has loads of photography and graphical content if that's your preference - but you can most definitely interact between them. This should be a key factor as the fediverse matures and should help to ensure that our communities continue to grow.