Opinion: The Death Adder only has 5 buttons and is not a very good shape. Plus it costs more. The only argueable "advantage" is the software. I know many people from the Logitech MX crowd that would consider it a downgrade.
I do agree that the Lachesis is a high risk. Especially with the cost. But as you say if he wants a major change it is a good candidate. But he could easily end up unhappy.
Compare the actual benchmarks.
We can banter about subjective opinion all day long, but it won't get us anywhere.
As long as we are bandying about opinion:
The death adder shape is far superior to the lachesis shape. It's larger, is more comfortable and is designed to fit the right hand. In comparison, the lachesis is the typical tiny razer mouse and works for either hand.
In terms of sole benchmarks, the razer mice beat the logitech series. Although, we all know how much personal preference comes into play with mice. It's like cars - Sure the Nissan GTR performs better than a lot of cars, but some people still find it butt ugly. The person won't drive an ugly car, thus making the car worthless.
Pathetic analogy, I know, but it conveys my point.
More or less, you should be looking at the 518, Death Adder, and Lachesis. Any mice beyond those is pretty worthless.
Except for the intellimouse 3.0. Gawd that thing was amazing.
Edit:
The Death Adder is a great shape. It is very similar to the mx 518.
Yes, it is very similar. The design of the Death Adder was a combination of the 518 shape and the Intellimouse shape. Sadly they combined to create an annoying knob in the middle, but it's still a nice blend.