They're both hard hitting rounds... if you don't reload, i'd consider the cost and variety of avaliable ammo. That's why I like the 270 better than 280 - variety and cost. both are similar rounds.

The .45-70 as a factory round from the big three (Remington, Winchester, Federal) has a bit of a dropoff compared to the .450 Marlin. This is only because they have to load them to lower pressures so they are safe in any .45-70. When loaded properly to the pressures that the Marlin rifles can handle it becomes nearly identical to the .450 Marlin. The low-power factory rounds still have enough pop to handle deer and black bear. If you are going after larger game, Cor-Bon and Buffalo Bore make high-power rounds. Or if you reload you can control the whallop yourself.
In this respect the .45-70 has a little more versatility. The low-power factory rounds are cheaper and more widely available than the .450 Marlin ammo, but if you go with the Cor-Bon in .45-70, it may be a couple dollars more per box for the .45-70.
I also heard rumors a long time ago about the belted rim of the .450 getting caught in the ejection port, but that was when it was first introduced. I believe they've widened the port to adjust for this and it is no longer an issue.
My personal choice would be the .45-70, but it all depends on your needs.

I got one of the 450 marlins early this year. I really like the rifle but wish I would have gone with the 45-70 because Hornady is the only company making 450 ammo. The 45-70 has all types of rounds avaliable for it.

My brother has an 1895 in .45-70. The low power factory rounds are a joy to shoot, but we tried some of those Cor-Bons, and man, you don't want to shoot them all day. I would imagine the .450 has a bit of recoil as well.

I have a 1895 guide gun in 45/70 and also 450. I take the 45/70 all the time as I use the Garrett Hammerhead ammo Check out the at
http://www.gunforums.com/ubbthreads/showflat.php?Cat=&Number=5432&Main=74

http://www.garrettcartridges.com/reviews2.asp