I assume you are talking about the Federal blue box load with the 300gr Speer Hot Core bullet?I prefer the 300 grain bullet for the flatter trajectory and expansion. The Remington bullet really opens up on deer, so if you shoot one of these hollow points keep off of the shoulder. Rib cage is fine.
Although I reload, I recently went to the Speer as it holds together better than a deep JHP such as the Remington or the Winchester bullets. My reloads run just a tad over 2000 fps but factory ammo will be a bit less. Based upon my reloads, I would opt for the Federal ammo using the Speer bullets.
I'll be the fly in the ointment here.
This is in part simply for the fact that the 45/70 is not a long range hunting rifle when compared to many other center fire rifles, so why go there.
Why not optimize this grand Ol'Caliber for what it is and what it has done so very well since 1873.
Any gains seen with the light for caliber bullets, jacketed or cast, are never enough to put it into the long range category so seriously look at Wide Flat Nose cast bullet loads with bullets of 400 ++ grains weight and velocities in the range of 1500 - 1700fps.
My 465gr WFN cast at 1650fps is EVER SO EFFECTIVE, does not kick me into next week, is very deadly and does not cause the excessive meat loss seen with some of the short on integrity jacketed bullets or the WFN cast bullets of 350 +/- grains at velocities of over 2000fps.
It is also good for 200+ yds.
I first hunted with my 45/70 using a 350gr WFN cast at 2300fps and it only took one time seeing that huge hole to question my choice.
Enjoy the 45/70 for what it is and don't try to make it what it never will be, no matter how light the bullet or how high the STARTING velocity. Those light for caliber bullets shed velocity and energy like water off a ducks back, decreasing what at first might appear to be the ability to increase the effective kill range of the 45/70.
Crusty Deary Ol'Coot