Equally punishable, but separately recorded.
Rape is a crime of male power and misogyny, as, I'm sure is penetration with an object. There is a difference, though (rape can cause pregnancy and infect people with STIs and viruses such as AIDS, for instance), and criminologists need to know the difference in the figures and the motivation. I think it would be highly distasteful for any of us to even start wondering which is worse.
I'm not sure of the motivation for women sexually assaulting other women. Clearly it comes from a very dark place, but it is, fortunately, rare, and conflating it with rape is never going to be helpful. Recording it as a separate crime will allow research to be more meaningful.
Those who want to see the crimes lumped together - what would be the point, do you think?