Only 2 major differences!
The aluminum Series 11 uses new Ion-X glass on its display, which Apple says is twice as resistant to scratches as the aluminum Series 10's glass. Otherwise, they're both water-resistant to 50 meters and IP6X dust-resistant, and the titanium versions of both watches use the same sapphire crystal instead of Ion-X glass.
and the new series 11 can run upto 43 hours instead of 36 hours on 10.