Surf is built entirely on editable WYSIWYG documents, NotebookLM's main AI is built on chat. Surf is built to be a bit more open, NotebookLM was a bit locked down for our taste.
An example I'd highlight is taking notes against a PDF.
NotebookLM will convert the PDF to simple text, and the chat responses are read only. NotebookLM also has a lot of strict walls between chat, artifacts & sources. You have to "save" responses as (read only) notes, and move notes to sources.
With Surf you can generate notes that deep link to specific pages in the PDF, and Surf will open those pages in the original PDF. You can remove the fluff you don't want in your notes. The intention is to be a little more open -- all notes are sources from the get go, you don't have to save or migrate anything.