Solid copper or stranded copper for speaker cables? What is your choice and why?

I choose speaker cables by how they sound in my system.

I learned long ago that high end cables make a huge difference in the sound of my system over just wire. Also, there are many parameters that determine the sound quality of what you use, like wire composition, metallurgy of wire (purity, crystal length… etc) diameter, strand count, geometry, dielectrics used, etc.

If you’re just starting out playing with conductors, and you would like to pursue it. I recommend going down to Home Depot and buying a 12 gauge three wire extension cord. Cut off the plugs and use this as speaker wire. These are multi-stranded. Obviously just use the black and white conductors. This would make for a good standard to which you can compare other wires. It is surprisingly good sounding. Better than lamp cord and inexpensive speaker wire. Although generally very inferior to purpose built high end audio speaker cables.

Oh yes. I have hear speaker cable made of solid core, square stock, multi stranded and combinations. Which one sounded Best depended on all the other parameters taken together.

There are always pros and cons with each type (of most things in audio).  I'd think a hybrid of both (or multiple sizes) might be the sweet spot.  But also "as always", it's subjective, and there are variables that are unique to every situation. 

solids do better in general, but there are inconveniences of inflexibility especially if you intend to disconnect and move your components around.

Stranded.  Because sold 10 awg wires are not flexible.  Jerry

Of those I have tried, my favorites are made from multiple strands of individually insulated, relatively small gauge (i.e., 20-24 awg), solid core wires.