I own a mx518 and a XAI, and I have an IME 3.0 being delivered soon, but between the two I currently own, I'd take the xai over the mx518. The only drawback to the xai is the slight built in acceleration and the fact that the sensor is very sensitive and dust can get in there easily, which disrupts the tracking. But as long as you keep it clean, you'll be good. It's extremely comfortable as well. The reason why I'd take it over the mx518 is probably just because the shape fits me better, it glides more easily along my cloth pad and I can adjust the settings of the mouse using the LCD on the bottom of the mouse.
However, the mx518 is an extremely good budget mouse, which will be cheaper than the IME3.0 now. The only possible downside is the lack of on-board memory and the fact that the mouse has built in prediction (which is preference to be honest).
So I'd go with whatever your wallet allows