I've used both of these before (currently have the DeathAdder), and I'd recommend the DeathAdder. The only advantage to the Mamba is that it has the option to be used wirelessly, but I found that the battery life degraded quickly and after 2-3 months of use, I had to leave it wired in almost all of the time. The DeathAdder is a nearly identical mouse, minus the wireless option, but it also uses the Synapse 2.0 software which allows you to configure macros and store those settings on the cloud so they'll automatically apply if you use the mouse on multiple computers. The Mamba has similar software, but it won't sync to the cloud so you have to configure it on each individual PC you use it on.
As far as size, number of buttons, and general feel go, these 2 mice are almost the same. The only other issue I noticed was that the grip pads on the sides of the Mamba became detached after a month or so of getting it, but the DeathAdder's have remained in place for almost 6 months now with no problems.