I love both the Deathadder and the G500 and have played on both for quite a while.
Deathadder is basic, with the normal mouse set up and two side buttons which is nice for going back on webpages. It feels perfect in your hand, not too heavy or light and is just sexy in general and a great mouse.
The G500 has weights to increase or decrease the weight. Not all that great, since once you get comfortable you never change it.
It has an open mouse wheel button that is great for flying through long PDF/documents/sites. A simple tap of the button returns to precise wheel clicks.
And my favorite feature is the on-the-fly sensitivity change buttons that comes with a little LED area so you know where you're at. On my desktop and CSS, I have the normal 3 bars of sensitivity. On Killing Floor, I click down twice so I'm at 1 bar of sensitivity. We've all played a new game where the sensitivity is way the hell too high, having this mouse just makes it so you don't have to go into any options, just a few clicks and you're good to go.
You really can't go wrong with either. I'm just enjoying the G500 I got on sale for 30 bucks. haha