Osimhen for me. Kane's became visibly slower already and I don't trust his ankles one bit even though on paper he's only 29. And while he compensates for it with his brilliant playmaking, he's also not much of a use in a high-pressing system and I don't really want for ten Hag to compromise his system for a star player's convenience.
Osimhen, on the other hand, is younger, works quite hard off the ball, is rapidly quick, very strong and fantastic in the air. The only thing that Kane really has over him, experience aside, is his playmaking - but Osimhen is actually quite decent in the build up himself.