I have some experience with both calibers. I used a 12" Encore in 460 S&W for a while, as well as a 12" Contender in 45/70 before moving up to the 10" BFR in 45/70. I felt that the 45/70 was more versitile, so I ended up selling the 460.
In the BFR, I can run everything from super light loads with Trail Boss and a cast bullet, to first class bone crushers using 500gr or heavier loads running full power. 45/70 data is available everywhere from pretty much every sorce that offers data. Although, the 460 has plenty of data out there as well. Especially if you count 45 Colt and 454 Casull data in there.
The biggest advantages the 45/70 has in my mind, are the lower pressure and heavier bullets. I just wasn't crazy about running a handgun way over the 50,000 PSI mark. Or running the shorter brass in the 460 chamber for that matter. And I preffer bullets over 400gr. My favorite in the 45/70 is the CP 440 LFNGC.
As far as which one is cheaper to feed, that one probably goes to the 460. Brass and bullets both seem to be a little bit cheaper. But the 460 does burn a little extra powder.
Since the guns chambered in either caliber are going to be equally large and cumbersom, just pick the style that you like. If you want a double action or a shorter barrel, that leaves you with the 460. If single action is more your style, you can have either caliber in the BFR.
In a somewhat related story, I was at Cabelas looking at a 7.5" octogon barreled BFR in 45/70 yesterday. I just listened patiently as the salesman told me that this gun was really more than anyone can handle accurately, even with the cowboy loads. He then informed me that it is really just a 460 S&W super mag, and that you can shoot 45/70, 460, 454, and 45 Colt out of it. When I told him that I actually owned one and that this was not only ingnorant, but one of the stupidest things I'd ever heard, he became upset. He took the gun from my hands, litteraly tossed it back into the case, and walked away mumbling something about a stupid kid wasting his time.
Andrew