Nitrus and Karma have very little in common. Nitrus is based on the S Series firmware with additional features, where as Karma is an entirely different source base. Nitrus has a relatively small mono screen where as Karma has a large greyscale screen - there is a family resemblance but Karma is more feature packed.

Even so, Nitrus is very feature packed for a player of its size and battery life.

Eigen is a different industrial design for different retail channels. It is functionally identical to Nitrus, so just choose the one you like the look of (for me that would be Nitrus).

Rob