Thanks for the links.
Back to the Parthian infantry, I did some research. Somebody on the Total War Heaven forum did a one-on-one on Medium difficulty. It took an experience+2 unit of his Hillmen to beat a standard unit of AI EI, and then it was close, the extra men make the difference. By the screenshot he posted, Hillmen vs EI: Attack:5-3 Charge Bonus:2-1 Defense:9-10. Eastern Infantry have 120 men, poor morale, can sap, bonus vs cav. Hillmen have 80 men, bonus fighting cav, can sap, can hide in long grass, fast-moving, may charge without orders, good stamina. The one-on-one says it all, though. EI also did better against all other units tested: Barb. Cav, Swordsmen, Scythe Chariots, Archers, Militia Hoplites. So yes, EI are better. They cost 40 more, but upkeep is 20 less, so you get more for your money. He also tried 3-3, and EI still won.