That's a hard choice right there.
This is the Benq
What I like about the Benq is the scroll wheel mainly: it is VERY precise and comfy. Accuracy is okay, not bad for a wireless optical mouse. Also, if you click all 3 buttons together at once, it shuts off the mouse. If you leave it sit idle for like 10sec or something, it also deactivates and you just hit a button or the scroll wheel to turn back on.
The V500 is more comfortable to use. It is wider and angled upwards, whereas the Benq is pretty narrow and lays flat. You would have to have small hands to really be comfy w/the Benq; I have big hands and am barely okay with it after a week or two of use.
Also, the V500 is more accurate & precise. As you'll notice from the V500 review, it is much more suitable for applications where preciseness is necessary; like photoshop and stuf.
The V500 is definitely higher quality, but also higher price. The V500 retails for $80 and the Benq $30-$40. Find the best price of each and make your decision accordingly