Latest posts , because it places focus where it should be: on the latest activity. Category view is very handy to understand categories and how they relate, but it’s less likely that people will benefit from bumping into each other. Default to latest posts and let people navigate to categories as needed. That’s my $0.02.
For now we have moved the default to Latest, as well as added links at the top to quickly switch between Unread tracked topics, Starred topics, and the Categories view.
There’s still lots of discussion in the Discourse project about making this a user-definable preference in some way, so the UX may change again at some point.