Userbase may or may not be important to you as a user. Since they’re federated services, a Firefish user can follow and interact with a Mastodon user and vice versa. In terms of content alone, there’s effectively no difference. You can still subscribe, read, reply, favorite/star, retoot/boost, etc…

However, a very popular software package in this sort of ecosystem will have the ability to set defacto standards. If Mastodon changes something in the way their software works that isn’t explicitly defined by the activitypub protocol, other software developers will have to choose whether or not to adopt and implement similar changes.

They don’t have to, but they risk being seen as incompatible or not having a feature or behavior that users expect if they don’t. A great example of this are all the slight differences between Lemmy and Kbin. Kbin tries to do its own thing in some ways but some users expect feature parity with the larger Lemmy.

I personally like Firefish (formerly Calckey) way more than Mastodon. I follow and interact with plenty of Mastodon accounts. Right now, it isn’t particularly important to me that the platforms aren’t exactly the same, the core activitypub based microblogging platform functionality gives me what I’m looking for.