IIRC, the MkVIIs (.357mag, .41mag, and .44mag) required slide/barrel/spring/mag assemblies to change between calibers. The .50AE MkVII was the basis for the MkXIX. All calibers (minus the dropped .41mag) can be interchanged with just the barrel/spring/mag (and the bolt for the .357mag).
By getting a .44mag MkVII, you would need an entire upper assembly to switch calibers. With a MkXIX, you would not need to get a new slide, Just barrel, spring, mag, (bolt). Plus, the MkVII .50AE and the MkXIXs have the Picatinny-type rail integrally machined on top of the barrel for scope mounting. For the other MkVIIs, you need to attach a Picatinny/Weaver-type rail onto the dovetail rib on top of the barrel.
In short, you save the cost of a new slide if you want to change calibers and decide to go with the MkXIX.