solid core VS stranded for inside speaker cabinet

I recently purchased a crossover upgrade DIY from a popular seller GR research for a Klip 504c however the seller sent me in the kit Solid core wire as part of the kit   , I always thought stranded was best and I haven't really seen solid used before inside of a speaker cabinet , I was taught that electrons flow on the outside of a wire not the inside so having many strands is better vs 1 16 awg solid core wire , I thought that this seller maybe just wanted to save a little money by sending this wire out but he insists solid core is better , my gut wants me to finish this project off with stranded not solid core , I want to ask what's better and why,  is it possible solid wire from the cross over to the speaker is better or is stranded better from the cross over to the speaker?