I disagree with Oneoldsap. I understand the topic to be about the respective cartridges, not rifles that chamber them. With that in mind, I see the .30-40 as the better cartridge since it can shoot about 200 fps faster than the .30-30 at similar pressures as Eastbank noted. Put it in a modern rifle like the Ruger #3carbine (it was a standard chambering for the gun) and you can push it to 50,000 psi or so. Of course there are a heck of a lot more guns chambered for the .30-30, but that is not what this topic is about.
Similarly, a safe load in a .45-70 Trapdoor Springfield is not as hot as a safe load in a Ruger or Marlin, so you cannot argue that the .45-70 is not as effective as perhaps a .450 Marlin if you only use the lower presure load for the Springfield as a comparison.