I was taught that solid core held the signal better over distance but is kind of brittle, so it's best used in conduit runs. Stranded is more flexible and better suited for patch cables or other uses where twisting and bending is more likely.
Worked at one place where the cheap bastards had a reel of solid they used to make all their patch cables which they then weaved into a horrid, tangled mess. I wound up having to replace about 80% of them.