Two routes to the same deliverable.
Classic link building is procurement. You prospect sites, negotiate placements (guest posts, niche edits, resource page listings), and pay or trade for insertion through services built around existing inventory. Digital PR is earned media: you create something a journalist wants (a reactive comment, an expert quote, an original dataset) and the link arrives inside coverage they chose to write. The digital PR route starts with a story, not a target list.
Framed that way, digital PR vs link building is really a question about what your links need to sit inside.