Another snippet on this topic posted by @minrk on gitter earlier today:
filepath is okay, and usually better, because it encodes the location of the file within the repo, which binder then resolves to a url. This doesn’t, however, allow specifying url-specific things like params, so you have to fallback on urlpath if you want to do url things. Any and all url things should be supported, though. The url for a notebook is /notebooks/ not /tree/, and apparently the redirect from /tree to /notebooks doesn’t preserve params, which is why your link isn’t working.