Firefish is more tumblr-y, mastodon more twitter-y. Both are good, have their pros and cons. It's a matter of preference. Firefish is extremely easy to set up though, like ridiculously so.
Just started using it. First impressions, it's definitely feels more unique than mastodon, and has features its missing. Like there are reactions in addition to favoriting a post, and you can quote post. At this point, It kinda feel's like Mastodon's kbin.
A mastodon DM is a private post. Setting a post to private before sending means only mentioned accounts can see it, and all mentioned accounts get notified. In a sense this enables both DMs and group messages too. But it’s not the most elegant interface for it tbf. A good app makes this feel more DM like, such as Ice Cubes on iOS or the equivalent on Android.
I find Firefish adorable for some reason? Like I know nobody who uses it but when I signed up a bunch of random people made sure to greet me. It was very sweet.