The Deathadder is BARELY any better in the tests that I've seen (sorry, I don't have a source, it was a whiiile ago) using software that measures latency, perfect control rate, and stuff like that.
And it really only mattered if you played on a really high sensitivity. So the Deathadder and MX518 are pretty similar.
FYI, if you use W7 or Vista, you'll want to take a look at this http://www.ngohq.com/news/15043-how-to-increase-usb-sample-rate-in-windows-vista-7-a.html
I did it, and noticed a substantial difference afterwords. I knew my mouse didn't feel quite right after using it on XP.
Here's a picture of my mouse btw (just took). As you can see the logo where my palm sits is fading away, but besides that no issues.
And for fun my keyboard
Not too fond of my mousepad, I wish it was a bit more buttery, but it's a Razer Goliathus.