Lee,
I think it's more a matter of preference than actual
(practical) differences between stranded and solid wire.
There is the possibility of interaction between the strands
of stranded wire (corrosion, smog, 'stuff'), no idea if it
would be objectionable in your area. One 'alternative' if
you decide on stranded wire is to use insulated wire and
seal it where you have to make connections. Ought'a make it
last longer before any of the 'interaction' between strands
begins.
You already have a fairly good idea of how long Copperweld
will last. Other types of wire will probably last about as
long, sort of.
The diameter of the conductor at HF is going to make very
little (if any) practical difference in length. You will still
have to do the normal 'checking and chopping' for resonance and
that will take care of any differences because of diameter. It's
frequency dependent, the higher the frequency, the more difference
it will make (but we're talking huge differences in frequency, not
Khz or Mhz differences).
The 'best' kind of wire to use is whatever you happen to have
the most of...
'Doc