For me, I chose mastodon because it was the most complete and professional looking that I knew of.
I like the design choices of longer but not massive message length, no quote toots, etc.
I'd probably choose it again, but I'd also shop around more seriously first.
I kind of regret the domain name though, I think it would be better to not tie so closely to the software.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!