Better is a matter of opinion.
The main difference is in features. Firefisg has more things you can use to arrange what you see. It has longer text limits (usually). It has a more developed web UI.
Mastodon has better apps in Megalodon and moshidon (opinion) compared to the best calckey/firefish app I've found, milktea. Firefish used to be called calckey, but changed its name.
In my opinion, if you want a simpler, hashtag driven microblogging service, mastodon is king. It does what it does very simply, and that can be a very good thing. It's what I prefer for most things.
If you want a more customizable service with a longer form writing capability (but still shorter than lemmy/kbin lengths, firefish is awesome.
I tend to use mastodon on my author account and firefish for poo and giggles. The established # based activity is usually easier to interact with on mastodon because of the lower character limit. Things stay concise and smooth until someone from other federated services takes part.
With mastodon being the better known service, you'll also get nore active "local" feeds. This is rarely something I use since I'm not looking for that type of interaction at all. As an author, I look for ways to get my stuff out there without shilling, and the network of writers and readers in the various # groupings is very good at that. Yeah, you can interact with those from firefish but you fuck up the flow.
