For me, it is a descsion based on what i’m going to use the bike for.
For just general pootling around, not on steep or gnarly stuff, cheap rim brakes will be better than cheap (cable operated) disc brakes (because the braking surface is at a larger radius, the clamp force can be a lot less, and as such, things like high friction in the cables/calipers/levers etc has less effect).
BUT,
If you think at any point you might want to do some ‘proper’ mountain biking, i.e muddy, steep, rocky etc, where you need repeatable braking and control, then at the very least a bike with disc/caliper mounts would be a good idea as other have mentioned.
Even for a cheap (and hence heavy frame) some sensible second hand sourced upgrades to a better fork and hydraulic discs can release a lot of intrinsic capability for little extra cost!