The 450 Marlin gives better performance than all but probably 1% of 45/70 loads from very few select rifles. And you don't have to worry that one of those 458 WM level loads will blow up someones rifle by mistake. For any but the most serious reloader the 450 Marlin is the better choice for top 45/70 performance, especially if you take the prices of 45/70 custom loads that outperform the 450 Marlin.
I used to love being able to hit hard at 1000 yards. As I get older I find hitting a mini ram at 200 yards with the 22 oddly more satisfying.