Go to www.hodgdon.com
They will have plenty of load date for you in most bullet weights.
The 500 S&W will get you +1600 fps with the 440 gr hard cast bullets. Some 500 gr loads will get you very close to 1500 fps.
Hodgdon lists the 45-70 in a handgun as a 1200-1300 fps round in a handgun with a 485 gr bullet and under 1500 fps with a 405 gr bullet.
Now I am not saying the 45-70 can not be loaded to higher levels but it is not designed to be loaded that hot, the 500 is designed for these pressures.
THere are two schools of thought as to which would be better. THe 45-70 and 450 marlin are identical, what one will do so will the other with similiar loads so we will just compare the 45-70 to the 500.
Some feel that the higher Sectional density bullets for the 45 cal for a given bullet weight will penetrate deeper. THis is true to some degree but if you use a quality hard cast bullet, the difference will be very little.
Others feel the larger frontal area of the 50 cal will have more authoritative effect on big game. Again, this is true to some degree but remember that neither of these generate enough velocity to have any real hydrostatic shock wave.
Simply put, both will put a heavy hard cast through any animal on the planet. The 500 maybe better suited for handguns because of the powders used and the pressures its loaded to which both add up to more powder burnt in the cylinder which will make it more efficent and burn cleaner as well.
Its just to bad that the cylinder on the BFR is so long for the 500 S&W.
There are also bullets up to 700 grains for the 500 which I have read some load data that lists them at nearly 1400 fps with this bullet weight in the smith. THe BFR is at least as strong as the Smith.
That said, if you already have the 45-70, is getting tooled up to handload the 500 worth it? That is your call.