Who would win?

yeah...i say Ichi cuz of that "High Spirit Density" stuff and the fact that even Rukia is impressed by his skills as a Shinigami.

Obviously, Ichigo. For one, she doesn't even have Bankai.

However, I still say Rukia is pretty good. Her Shikai has multiple powers, and shes even able to defeat Arrancar with it, including an Espada.

Ichigo just wins automatically against normal Shinigami. Theres no comparision. With his Bankai and Hollow Mask, there isn't a chance for Rukia to win, even though she is pretty tough on her own.