The .444marlin bridges the gap between the .44mag and the .45-70, falling just a little on the .44mag side of dead center b/w the two...I personally don't have much use for it, I bought one and sold it within a year, it's definitely a good deal flatter shooting than the .44mag, and it handles heavier bullets a bit better, but the fact that it's loaded with handgun bullets (I don't know if anyone has begun producing a rifle bullet in .429/.430" yet, when I bought mine, they hadn't) really really limits its effective range to barely more than the .44mag. I wasn't impressed with it but it has its place, it'd be good for mid weight deer species (heavy mulies, elk, really light moose, close range sheep and goats etc) at relatively close ranges (under 150-200yrds, depending on shooter). Mine did quite well on whitetailed deer, ferral hogs, and elk (elk at very close ranges), other than the elk (which I've never used a .44mag for), I don't honestly know if the performance was any better than my .44mags, that's why I sold it.

If they've come out with "rifle bullets" in 44 diameter, then all of the above would change, I'd probably buy another one for black bear hunting and elk for certain.

Since you're a reloader, I'd recommend the .45-70, you can bump up the power of the .45-70 to beyond that of the .450M and brass is a bit cheaper (some people don't like reloading belted cartridges either)...non-reloaders are stuck with a little less power from "standard" factory rounds, or they can pay prices on par with .450M ammo and get .45-70+P loads that match or surpass the .450M, not to mention the +P rounds come in about as many bullet types as the .450M, which is quite limited in selection.

The .450M is a great round, it's not going to replace the .45-70 though, although it may share a relationship more or less on par with say a .308win and .30-06 in coming years (i.e. neither really has a distinct practical advantage over the other and both are quite popular). I bought one to see what it was like, I didn't see much difference in performance vs. my tried and true .45-70's.

Likely, if you're going to need more power than factory .45-70 ammo has, you're going to need a bigger increase in power than the .450M requires, and you're not going to need more than either on this continent (handloading the .45-70 to +P/marlin loads evens them out, so I was refering to factory loads). So picking either of them is up to you, whether you've got a 400grn bullet at 1900fps or 2100fps, dead is dead, either will be quite stout enough for anything you'll encounter in North America.

Pierce had a interesting article using a .45-70 in africa, shot a cape buffalo bull in the shoulder, it went through and through, and penetrated nearly through the cow standing unseen behind him (bullet stopped beneath the hide on the far side). The first shot would have been fatal for both, I don't know if he continued on the cow, but he gave the bull two or three more shots at the PH's instruction (shoot until they're down) that would have also been fatal alone. There's a interesting thread on this board about it, you might check it out.