What method should I use for a sidebar widget that reads latest posts?

Sounds like you don’t need the querying power taxonomies will offer, but it would make sense to use if you think you’ll frequently have the same authors publishing; reduces redundant data (i.e. lots of the same entries in post meta) and improves the UX for administrators.

Same applies for websites; if there’ll be many with rarely ever the same, stick with meta.

Either way, a custom post type seems like a good plan to me.