Matthews by a hair. The biggest differentiator is that Matthews' goal scoring comes from such a high variety of shot types and locations on the ice. So many of Ovechkin's goals were just him standing still on the PP and ripping one timers from top of the left circle. That's obviously impressive, but easier to game plan against.
Matthews can score goals at high volumes literally any way you can imagine. Deflections, tips, rebounds, dekes in tight, wrist & snap shots off the rush, one timers, dekes off the rush, slap shots, stationary wrist shots, backhanders from the slot, wraparounds... the dude may have the most well rounded goal scoring arsenal we've ever seen.