Downloading a new Google Spreadsheet in excel format - google-drive-api

Question

We are finding a new behavior in the export API between new google sheets and old google sheets.
It is also somewhat unclear exactly where the endpoints are supposed to be- my understanding is that everything should move to the drive API eventually.
So, what we are doing is first getting the information for the document using:
https://www.googleapis.com/drive/v2/files/
This works fine, and we get all the info.
Within that, we have the export links that look like this:
"exportLinks": {
"text/csv": "https://docs.google.com/spreadsheets/export?id=&exportFormat=csv",
"application/pdf": "https://docs.google.com/spreadsheets/export?id=&exportFormat=pdf",
"application/vnd.openxmlformats-officedocument.spreadsheetml.sheet": "https://docs.google.com/spreadsheets/export?id=&exportFormat=xlsx"
}
Directing us to use the docs.google.com endpoint, which we do.
When we use the "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" link (excel format) this works fine for OLD Google Sheets.
However, for new google sheets we get a redirect to an endpoint that is in our Google Apps domain of the format like this:
https://www.google.com/a/.com/ServiceLogin? etc.
We have put the OAuth token in the header, again, this works fine with old sheets but not with new sheets. If we remove the token, then it redirects us to yet another signin page.
Why is it redirecting rather than returning a 403 unauthorized if there is an issue with the authentication? Why only for NEW sheets not old?
This results in some HTML being returned, not the file- as if we were a browser.
So the question is two fold-
1) how do we download new sheets in excel format programmatically using Oauth
and 2) If its different than old sheets how do we detect which is which so we can support both?