I think Federer is going to play till he's 35, but will Rafa play till he's 30? Or will Rafa retire at 28-29 like other injury prone players like Rafter, Becker, Borg, Safin etc?
Borg wasn't injury-prone. He retired for other reasons. He also retired at 26, not at 28-29.
As for the question: Both will probably retire sooner than expected but all the injury timeouts from Nadal should logically buy him some time, not retire him faster. IF Nadal hadn't been injured so much it would be a bigger chance that he retired before Federer. Now probably not.