I think they are separated by around 1500 years, so I’m sure the Mongolian army would be scarier. But the Alexander-era Persians wouldn’t have that choice, right? For example stirrups and advances in composite bows (they’ve existed for a long time, but were high tech things, so I’m sure every culture iterated on the idea and 1500 years of iterations add up) probably made Mongolian horse archers a lot better than the options they had.