why in the world would you use the horses WS???
every other army that can put a character on a chariot has "replaces crew", chariots fight against the crews (or character) WS. the difference being that we cannot run the war Alter without AL so that standard notation is irrelevent. the horses are not crewing the Alter it is AL so you would use his WS for CC.
until a FAQ comes out that states "CC attackes against the War Alter use the horses WS" I will use AL's WS thank you.