One other consideration. At the moment I believe that the .450 Marlin is currently limited to one bullet.....the 350 Hornady. And even though that is a fairly good bullet....I would not trust it to give the kind of performance you can expect from bullets like the "sledgehammers" from Garrett. Without a doubt the Hornady bullet will work for game like deer, black bear, boar, and probably moose, and elk. But I don't think I would trust it entirely for a stopper on grizzly or Brown Bear. For that job I want a bullet "I KNOW" will hold together and penetrate.....not "think will". And bullets like the Sledgehammers and some of Buffalo Bore's bullets.....I KNOW will! And that is a very big difference indeed.
If as you said you only want it for whitetails....just about any factory load in either caliber will work! And the "Trapdoor loads" will get the job done for alot less money....and with many fewer bruises.....on both you....and the "eating meat". Of course if you handload...you can do the same with either. But it's nice to have the versatility in the factory loads as well.....especially if you find yourself accidently seperated from your handloads. And in many places the .450 Marlin will not be stocked.....the .45-70 is nearly always. Maybe an important consideration to you.....maybe not?!?
P.S. Hey...but what a nice dilemma to be faced with
! It wasn't that many years ago we did not have these kind of choices. It would have been much simpler......and not nearly as much fun! (I just went through this process for myself. Trying to determine which of those two calibers I was going to purchase as a "second rifle" for my wife and I when we hunt Alaska together. It will triple duty as one of the camp guns, (the other an 870 Remington with Brennekes), a "packing" gun for when we are packing out meat, and the "heavy caliber" to back up the lighter...flatter shooting rifle the "other" of us will be carrying at any given time. I finally decided on the Stainless Steel Guide Gun in .45-70 for all the reasons I enumerated in my several posts above!
Once again...good luck!