RE: 45-70 or 450 marlin?
If you were going to buy a Marlin lever, My vote would be for the 45-70. It can handle loads that match the .450, yet you can buy "dumbed-down" factory loads (300 grains at approx. 1800 fps) that wont kick hard at all, and won't ravage a whitetail quite as much as the .450.
I've only seen one factory loading for the .450 in the stores, and they shoot 350gr at around 2200. This load kicks you like a mule in my opinion, at least enough to be a safety concern in a treestand.
If needs be, you can get all sorts of beefed-up .45-70 ammo from Cor-bon, Buffalo Bore, Garrett, and some others that will easily do anything the .450 can do. Take a look at some of their stuff online, they will say what loads are appropriate (safe) in whichever gun you choose to buy.
I guess to sum it all up the .45-70 is just a lot more versatile.
PABigBore