i have a guide gun 45/70.
i shoot both the soft loaded and hot loaded ammo through it. the factory remington ammo is MUCH easier on your shoulder.
i have loaded up some that was within max load specs, that was so hot i didn't want to shoot it even with a muzzle break and a limbsaver recoil pad. the really hot loads actually are so hot that if you hit an animal with it at close range, the bullet will explode, instead of penetrate like it should. i talked to remington and hornady about that. so unless you KNOW you will be shooting @ over 100 yards, there is no reason to load that hot, ever. plus it is HARD on the gun, as well as your shoulder.
if you buy the guide gun, even before you shoot it, get a limbsaver recoil pad (p/n 10102) and install it. it will be the best thing you ever did.
the low pressure rounds do not kick all that bad. but the big hot and heavy loads. DO.
i read an article in handloader magazine where the author loaded up some non expanding rounds and went to africa with his 45/70. he shot some cape buffalo with it. he said that on one shot, he shot and killed one bull and the bullet continued on its way (total penetration) and went on to kill the cow (total penetration again) on the other side! two drt's with one shot!
he also shot a different cape buffalo in the chest, and the bullet exited out the rear end of the animal. total penetration lengthwise.
these can be a mean and nasty load, if you want them to be.
i wouldnt buy the 450 marlin only because of the ammunition limitations. the same with the 444. there is so much ammo out there for the 45/70, it is almost as good of a selection as the 30-06.
you can buy slow and mild, or hot and nasty. depending on what you want to do.
the choice is your of course, but i WOULD reccomend a marlin 45/70 in any configuration to anybody except the really meek.