For some reason I have wanted a .450 Marlin in a bolt action similar to the 458x2" but you can buy ammunition of the shelf in .450 Marlin.
If I had to pick in a Marlin Guide I would go with the 45/70. Ammunition seems easier to find. Plus the verity of loaded ammunition you can purchase. You can get loads safe for a trapdoor or all the way up to loads with hammers heads and punch bullets good for anything that walks the earth.