I have a 1894P 44 mag with Ballard rifling. It is disappointing in the accuracy department because of the 0.4317" groove diameter, which is typical for the Marlin barrels and the 38:1 twist which limits the bullet and velocity range. If you buy a new 44 mag. barrel from Marlin your probably going to get the same old, same old, even with Ballard rifling. It seems that the 44 barrel is the problem with Marlin. I'll bet that your 45-70 has the correct groove diameter, better twist and shoots good. Go figure.
I'm in the process of rebarreling with a 0.429" diameter (the correct diameter) with a 20:1 twist in octagonal barrel form. I'm confident in much improved accuracy. I'll let you know when the project is completed. In the mean time I'd stay away from Marlin with a new 44 mag barrel - it is not the microgroove.