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

Stranded.

Having worked in a metrology lab for an aerospace bearing manufacturer, an R&D department for a major gas &oil company, and in gas & oil operations with electronic controls and monitors protecting millions of dollars worth of equipment, if it's good enough for that duty, it's good enough for my stereo.

Belden, Canare, or other high quality cable.

FWIW, in industrial applications we almost never use solid conductor wire.  They have a tendency to fatigue and break at junctions.  Especially if your wire strippers put a good nick in the wire when you are working with it.  Thermocouples being an exception, but those have been replaced by thermistors and RTDs, for the most part.

Good thread.  Have a great day!

cat 8 info

I found a version of this with 22 awg, these are 26 awg.

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In any case, I ordered half black, half red, cut the connectors off, twisted the red/black pairs, added WBT type connectors, done.

Mihorn + good description agree +++++.

@elliottbnewcombjr 

Back in the CAT5 days, the DIY SCs involved a lot of braiding, stripping, and bleeding fingers.  As you pointed out one method could be to simply use them as-is, but it seems another opportunity might be to twist four individual CAT8 cables and then connect them in a star-quad geometry for lower inductance. 
Using 22awg wire the individual CAT8 cables would result in a 13awg cable size per bundle, or 10awg if connected using star-quad geometry.  This source would be enough for a pair of almost 30-foot cables for only $140 or, a pair of 15-foot, bi-wire cables (two full runs per side), each cable at 10awg.