Actually, the solid vs stranded is an interesting subject. Clearly in most protected environments, stranded is more flexible, easier to use, etc. But outdoors, stranded, if exposed to moisture is less tolerant and fails sooner. That is because each individual strand can be "attacked" on it's entire surface, as opposed to a single large conductor where most of the copper is "inside".
Adequately protecting stranded wire can involve anti-corrosion pastes, or "liquid rubber", and care should be taken that water cannot "wick" into the jacket (it will get into the spaces between the strands).
I have used very large gauge stranded wire, with fewer, larger diameter strands, like we wire up electric ovens and stoves here in the US.
The more common choice is "landscape wire" usually composed of many fine strands, and corrosion and oxidation can literally "eat it up".
So, in my opinion, solid large gauge wire lasts best, but is more trouble to wrestle into place. Flexibility is usually not a consideration, since your track is really not moving back and forth.
Greg