As someone who has recently purchased both, I actually like the Logitech better from an ergonomic point of view. All the buttons are within reach without weird hand contortions, and 2d-scrolling with the button-modifier is nice.
The Kensington (Expert Mouse 7.0) definitely has a larger ball, which could be nicer for precision work, but I find it heavy and exhausting after long periods of use, due to the ball's large inertia. By contrast, the Logitech is nice and light, which is ideal for gaming and quick mousing around. My pinky rests in an awkward position for right clicking (just at the upper tip of the right button) and the upper buttons require moving my entire hand to reach over the ball.
The scroll ring is a plus, and is "ratcheted" for stuff like weapon switching in games, but it feels a little bit cheap and it grinds slightly (making a very distinctive plastic-on-plastic noise). The angle of the mouse hasn't been an impediment for me, partly because I eschew the included crap-rest in favor of a standard wrist rest at a decent height.
Another plus for the Kensington is its smoothness, which is a major plus considering the Logitech's tendency to get "stuck" every once in a while (requiring a quick application of hair or forehead grease).
I suggest just purchasing one and trying it out. They're <$60 on Amazon at the moment.
