There are many reasons why you might do better with one caliber than with another. They are both inherently very accurate. Could be recoil, or you are hitting the sweet spot quicker in one than in the other, more shooting of one than the other, and just plain more practice. It's all good when this happens.
Tim's 355gr WFN BTB is a very good looking bullet, or try the 330 LFN DCG or 335 LFN. I've tried several BTB bullets in my 45 cals...WFN and LFN's. I like the looks of the LFN's so I ended up using the 45 cal, 450 gr LFN in my 458 American and 45-70 BC and crimp in the last grease groove to get the case OAL right for the SMLE action(same load so I can interchange the 458 Am and 45-70 in the SMLE also)...and the 525 WLN in 45-90 to 45-120 cases.
~72 gr H2O for the 444 M case and ~ 78 gr H2O for the 45-70 case...8-9% difference.
The 444 M is slightly smaller in OD and "usually" shoots lighter bullets at a higher velocity than the 45-70.
Energy increases at the velocity squared so the energy increases much faster as the velocity goes up, so the 444 Marlin does a very good job until you get to the point of diminishing returns where the smaller case can't hold enough powder to get heavier bullets moving fast enough to overcome the slightly larger dia, larger cased 45-70.
This is mostly academic as a big slug outta either one will "geterdone" most effectively.
I've never had a 444 Marlin and I can't figure out why...I have cases, dies, bullets etc, have reloaded and worked up loads for friends that have 444's, but everytime I decide to build/buy one something else came along and grabbed me and the 444 was forgotten. Go figure.
Most of the time the arguments as to which is "better", I think, are washes except when someone starts waving around bullets of 450 gr or heavier, then the 45-70 gets center stage...mostly because there are few 44 cal bullets over 400 gr commercially available.
The 44 cal actually has a slightly higher SD than 45 cal when comparing equal bullet weights, because the 44 cal will be slightly longer, which also means the BC will be slightly higher. But when you are comparing flying trash cans, this small fact becomes moot or nit-picking.
In reality, many 45-70 users load the lighter 300 gr bullets at velocities closer to 444 M velocities because recoil just hurts. We continually hear about the 45-70 knocking the dog whizzz out of the shooters, but most 444 M shooters complain very little.
The perception of recoil is highly subjective...it doesn't matter what the caliber...a 400 gr bullet at 2000 fs in a 8 lb gun is the same, but say it is a 45-70 and watch the eyes roll...in a "little" 444 M, it "ain no thaannng".
No matter...both of them are excellent prospects and good to go, will take any N.A. Game with the correct weight bullet and shooting distance, so you can't go wrong picking either one.
Pick your poison and enjoy.
Luck