I owned the 8x30 Habicht a little over a year ago so pardon my memory, but when I think about what it was like owning the 8x30 and comparing it to the 10x40, there are a couple things I have noticed.
Size: Obviously the 10x40 is bigger, but it is bigger in the right places. The objectives being larger and farther from the main body made it easier to accomodate objective covers from opticron, as mentioned above. It simply work perfectly. Another thing is that with the longer objectives, there is more space between them to stuff the strap, making it really easy to fit into the supplied case. I found with the short stubby objectives on the 8x30, it was hard to find a nice looking objective cover and to fit it all in a case with the strap. Also, with the 10x40 being longer, it sits perfectly flat on your chest while walking around while the 8x30 tended to tip into your body, which can be annoying if youre out in a mist and have a wet jacket. These are all very minor points, but points nonetheless that favor the 10x40.
The 8x30, being lighter and having less magnification, together with a wide-set body, had the most stable view of any binocular i have ever tried. Everytime I brought it to my face, it felt like it was on a tripod. It was much easier to notice the magical and sharp view. The 10x40 undoubtably has more shakes, but when you realize you're holding a 10x40, and it really only weighs 24 oz, you come away impressed with the image stability as well. The view on the 10x40? What else is there to say, its a Habicht. Cristal clear, very bright, sharp as a tac. With the rubber chinese eyecups attached, I can't say that it has less glare than the 8x30 as these accessory eyecups managed to do a fantastic job limiting glare in both the 8x30 and 10x40 (Swaro should really pay attention to this accessory as it has made viewing through the Habicht so much nicer, and it looks very neat and tidy).
Where the 8x30 takes off is field of view. 324 ft vs 408 ft is a noticeable difference. Panning with the 8x30 in the woods was much more "wow" for me as everything was captured. The 10x40 was more of a point and shoot kind of binocular. The extra magnification on the 10x however coupled with the dead-on sharpness certainly does a much better job yielding more detail.
Lastly, like Roger Vine, I am a fan of stargazing, and am always on the lookout for a binocular that can do both. I haven't had many clear nights, but compared to the 8x30, the 10x40 blew them out of the water. The larger exit pupil, with large objectives, and high magnification really brought start clusters to life. The Pleiades have never looked so magical. Obviously a bit more shaky, but something remarkable nonetheless. I did order a Nikon SE tripod adapter to use with the Habicht for the night sky which will hopefully add another dimension to my viewing experience.
Overall, very pleased with my purchase. Learned my lesson after selling the 8x30, that I will NOT be selling the 10x40. Will I go back and buy the 8x30 again? I don't know yet. I feel like if I were to go 8x30, it would have to be a pocket binocular, which the habicht simply isn't.
Moral of the story; there is no ONE perfect binocular. Trying to find that binocular is a disease not curable, but certainly treatable with each subsequent purchase.