Mastodon seems to be where most of the action is, and it's an all-round more pleasant experience (both from a usability and a people's behavior perspective).
The crossposter I'm using to duplicate my posts on twitter is going to die soon. And they pulled the plug on the mobile app I use to access Twitter as part of the great API cull, so I'm likely to look there less and less often. Weirdly, the desktop client I use is still working, probably because it hasn't been actively maintained since about 2015 and only three people actually use it. It's readily apparent that Elon wants to make Twitter as unpleasant to use as possible, so I'm voting with my feet.
Meanwhile,
https://bird.makeup/ allows you to follow twitter accounts (albeit in a read-only way) from Mastodon, which solves the @worldbollard @number10cat and assorted journalists problems.