carto increases by 10% all ur units and that effects ur army saves, while war horses only cavalry, and that`s why it`s good 4 sudden pillage attacks
Ok, take a calculator and do this:
Cost of time saved from pillages because of 3% boost x number of pillages - Cost of Horse 8
To calculate the first figure get your army upkeep, multiple it by 3 and get the 3% of the total.
If that result to you a positive number, then go for horse 8