I want a small amount of resistance (1/4 to 1/2 ohm [per ft]) to damp the high frequency resonance due to the ESL transformer's leakage inductance resonating with the capacitance of the ESL. The 11 gauge of the [Belden coax] 8214 has excessively low resistance so fails to damp this resonance. Therefore the high frequency response of an ESL is better with the 8237.
Both the 8214 and 8237 use polyethylene insulation. The fact that the 8214 is foamed, doesn't really change anything except its flexibility. And because 8214 is a considerably larger cable, it is actually stiffer than 8237, even though the 8237 uses solid insulation.
The 8214 has slightly lower capacitance because the conductors are further apart, since the whole cable is larger. But the difference is insignificant as both cables have extremely low capacitance.
I used to use cross-connected coax. I found that it had too much inductance for use with ESLs, so I limited it to use on conventional, magnetic woofers where it worked very well. But a "pure" coax works far better for ESLs.
[Primarily] what gives different cables their different [sonic] characteristics is how they load, and therefore alter, the frequency response of the passive crossover networks in your speakers.