Solid is what you use if you need the wire to be stiff, and it's in an environment where there's no vibration or flexing. If there is flexing or vibration, a solid core wire will fracture quickly.

The more flexing, the finer you want the strands to be.

If you can make sure the wire isn't going to be subject to flexing at all, you might get away with using solid, but IMO there would have to be a good reason to do so.