That depends on which efficiency you are after.
Do you want the move to be in the browser history? Then use the redirect(call).
If you don't need the move to be in the browser history, and want the fastest response,
call the other domain's functions from your current domain's app.
(You could probably still make back- and forwards- history work with javascript,
which I think some SPAs make use of.)
If you didn't need the current domain to do anything with the request, then the global (er, top-level) routes.py may be able to help.
/dps