I went through the same thing and read all I could about both chamberings before deciding on the 45-70. This was tough since I am a magnummaniac and like to have the latest and greatest. What I found though was that the .450 Marlin was a factory offering of what 45-70 handloaders had already been doing in modern rifles. The .450 Marlin allowed them to produce in essence a "45-70 magnum" without worrying about someone dropping the cartridge in an old Trapdoor and blowing it up. Being a handloader, I had to give the nod to the 45-70 due to availability and versatility.

Currently I am having good results with 48.5 gr. H-4198 and 350 gr. Hornady FP Interlocks. 10 shots through my Guide Gun chronographed 1934 to 1948 fps. I wish all loads were that consistent.