Historically, we've been using Priority to determine how likely we will be fixing a bug for a particular milestone. Having read about the EBS, we'd like to use the Project Backlog Order as well. How do these fields differ, esp in how they are used by EBS? Will we still need to set Priority?
Project Backlog is a plugin that is entirely separate from EBS. That is to say, Project Backlog has no influence on EBS whatsoever.
From the Project Backlog Plugin's description:
"The Project Backlog (often called the "Product Backlog") is central to Scrum and Agile project management practices. It's essentially a feature wish list sorted by order of implementation."
Of course, how you come up with the "order of implementation" is entirely up to you and may well be based on priority. There are a number of articles out there that describe how backlog order and priority are (or can be) related:
Here a Fog Creek, we use both the priority field and the Project Backlog to prioritize our work.