I didn't know the MX518 had swappable thumb buttons, I must have missed that! ...
It's not $70, I got it for $60 on newegg. Maybe it costs more than the MX518, because, oh, could it that it is a superior mouse? It is NOT the same as a copperhead. The design is totally different. It is made to be much more comfortable and ergonomic. This time around the mouse was made especially for people who use the mouse in their right hand, and as such it is able to have more comfort than a mouse that can be used in either hand such as the copperhead. It's body closely resembles the Microsoft Intellimouse 3.0, which was a supremely comfortable mouse, and it uses rubberized side grips and buttons.
But IMO the best feature that was added is the swappable side buttons, so you can get them just to your liking in accordance with where you place your thumb, and having the 6th and 7th buttons under the scroll wheel is a much better location than on the right side, as they can now be easily pressed with your index finger.