I am so on the fence of this. For the first time I have both recipes and need to make one or the other.
I could make both but I don't want to do that. I like having a rifle with an 8X scope for far away stuff and don't want to carry a .44 and an SMG and the rifle.
I greatly prefer the SMG. I always struggle to find mods that can even work on a magnum, whereas the SMG works with a lot of really good ones, including the drum mod and the silencer mod. Not to mention it's way easier to get ammo for it.
Another fan of the SMG. The Magnum reloads much slower and the ammo counts out of the gate are higher on the SMG. Also, I find it much easier to find 9mm ammo than .44. I save the Magnum for late game stage play.
If you're going to carry a rifle in any case, go with the SMG. The .44 and the rifles pretty much have the same uses, and where the magnum would outperform the rifle, the SMG is going to be even better.
I like the SMG for daily looting trips and things like that. The Magnum has always felt too limited with only 6 bullets when you are in a pinch and have 3-4 zombies in your face after you fell through a trap floor or something. Using the Magnum, you would have to go for head shots and risk missing and wasting the opportunity, with the SMG you can spray and pray to get you breathing room to be able to finish the zombies off.