RE: 450 Marlin vs 45-70
 

I own the guide gun in .45-70 and I just love it. Especially when I'm hiking those mtns. in North Central PA. I harvested a nice bear with it 2 years ago - the 300 gr Hornady bullet did a fine job. Good handloads can be used with the Marlin too as it is a lot stornger than the older trapdoor guns. You probably can squeeze a bit more out of the 450 but I don't think any critter would notice the difference. And if you don't handload, you could have trouble finding ammo for the 450. The ammo for the 45-70 is more readilay available.
Just a note: I took the scope off my guide gun and replaced it with a Williams peep sight. It lightened up the rifle considerably and out to 50 yards I can shoot it as good as a scope. Even at 100 yds it is surprisingly accurate. I'd go with the 45-70.

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RE: 450 Marlin vs 45-70
 

Danny's got it right about non-handloaders versatility...right now, even the factory loaded .45-70 is more verastile than the .450. There are only a limited number of rounds for the .450, but truth be told, there are at least that many .45-70+P rounds available from factories as well, in addition to the countless standard loadings that are available.

Maybe the .450M can do slightly more because it's more powerful on the whole (based on average loadings from the factory only), but the limited bullet selection makes that difference more important that it is...i.e. maybe you can't find a .450M with a bullet suited to hunting grizzly bears, while there are .45-70 loaded bullets loaded properly for them that are weaker-you might be able to get away with hunting with slightly inferior bullet choices because of your slightly superior energy, but your terminal effects will likely be identical.

RE: 450 Marlin vs 45-70
 

I have the 45/70, my dad has a .450. They are both awesome guns. For bear and deer, I use the Remington 405 grain JSP. It is loaded conservatively, but the bullet is great and the speed is just fine for those applications.