I have found the Khurusan range to be very variable, a result of using different sculptors. So some Khurusan models are bigger, others smaller, some are beautifully sculpted, others, not so much. I haven't sent their Parthians, but I have seen the Xyston ones, and I think these are lovely models. Another option is Forged in Battle's War and Empire range, but I am not that impressed by the horse barding. Essex aren't bad, but they are Essex.
If I was starting a Parthian army, I would choose Xyston. As has been pointed out, the Khurusan price and shipping is a killer, and I think the Xyston ones look better.