Naturally, since its a hundred years older, you will find more variety of factory loads for the 45-70 than the .450..there are a lot more 45-70's out there, which means more ammo sales. more gushops stocking the ammo, etc... Do you load your own ammo? If so, i personally would not be quite as concerned as to the availability of factory loads. About half of the barrels i own are chambered in wildcats anyway.
In a short barrel or a long one, the .450 is still going to beat out the 45/70. It is loaded to higher chamber pressure, so it is going to produce more velocity in most barrels, and also more muzzle blast, recoil, etc. As to how much more...i doubt you would notice the difference when shooting at game.. i have a 5 1/2 pound .350 magnum from the Remington Custom shop that is a terror off the bench, but i dont remember the recoil when hunting with it!
With light bullets , i doubt the difference would be a great one, with heavier bullets the .450 may have a bit more of an edge... If i remember correctly, i read an article a long time ago about the original testing of the 45/70 back a century ago when the military used it, and it involved shooting through somthing like 15 1" thick pine boards at 500 yards! In my Ruger #1 45-70, i load it up stout, and them big bullets work! I have only killed 4 deer & 1 hog with the 45/70, (2 deer with Contenders & the others with the #1), but those big bullets really make 'em spring a leak.
When it comes to bigger calibers, i personally like the longer barrels, say maybe 14 to 15"..that is a personal thing, i think they handle better, and i seem to shoot them better. My choice would be a stainless .450 , 15" overall length (including the brake). I dislike the noise from muzzle brakes, but they do help recoil, and the best way to get familiar with a barrel is to shoot it, the more the better. If you cant stand to shoot the barrel because of recoil, then it is not a barrel you are going to be absolutely secure in using...no matter what caliber, if you dont shoot it well, and dont have confidence in it, it aint gonna do the job! I try to shoot my big bores regularly, maybe not a whole lot, but at least some every time i go to the range...
anyway, i think with either one, or quite a few other calibers for that matter, you wont go wrong if you shoot straight & put those big bullets where they are supposed to go. Also, if interested, i sent a few Graybeard members a copy of Layne Simpson's article on the 45/70 and they seemed to find it useful..be happy to do the same for you if you want it!
Good shootin'!