Tau don't even come close to Eldar tech. Just look at the weaponry. The pinnacle of Tau firepower is the railgun (hyper-accelerated magnetic propulsion) ... the Eldar use this for their basic weapon that just about everyone gets: Shurikan weaponry.
Wraithcannons/Distort-Cannons open a stable warp-hole that will completely obliterate anything it hits. Tau get headaches trying to comprehend that the Warp even exists.
Eldar armour responds to impacts and re-inforces the area hit. Tau armour is still just solid material.
Just to add to what I said, the Dark Eldar have an even better grasp of technology then Craftworld Eldar. Their vehicles and weapons all utilise their technology on a far more sophisticated level.
Last edited by Pariah Mk.231; 03-27-08 at 12:39 PM.