Who's more advanced technologically, eldar or tau? Not in terms of game rules or anything but in fluff.
This is a fairly ambiguous question. It's like asking whether or not Japan or America is more advanced in Technologies. Japan is by far ahead in the field of Robotics, but America is ahead in the field of Silcon.
Both the Tau and the Eldar are advanced in various ways, each having certain advantages over the other. While Eldar have a better understanding of the Warp and can utilize it to greater extents, the Tau's knowledge of Mecha far surpasses anything in the galaxy, save with the guarded knowledge of the Machine Cult.
Tyranid aren't exactly advanced, since their "technologies" reside primarily in bio-engineering, exceeding every other race we know of. But a Venon Cannon will tear through infantry as easily as a rail-gun.
One also has to consider Orks, who inherently know how to build, operate and loot/modify other races vehicles. They could construct their own Mecha, albieght with their own unique dangerous, that would work just as well as Tau Mecha, in Orc opinion anyway.
As far as Eldar "magic" is concerned, anything beyond what we understand is usually attributed to magic. Travel back to the age of smithing with a blow tourch and they'd thinkg you were the devil himself. "Singing" could easily be considered harmonically based/tuned technologies. (Kind of nifty if you think about it)