As original poster of this thread, I want to try to answer my own question. I tried the 8x30 (twice), the 10x40 and the 7x42.
I kept the 7x42. Why? These were the most comfortable in use for me. All the Habichts have a slow focuser. I loosened them a bit, by moving the focuser a couple of nights while watching a movie and it helped, a bit. Still not really fast focusers. However, it is less of an issue with the 7x42, because it has the largest DOF and there isn't a lot of need for fast focusing. One reason I kept the 7x42.
Another reason is the large exit pupil. Habicht is known for being one of the brightest binoculars available, with a stated 96% transmission, and to get the most of this feature a large exit pupil makes sense. The Habicht 7x42 is my low light glass.
The 7x42 has the least glare of the three, although the glare of the 8x30 didn't really bother me much.
The 7x42 has the largest eye relief, although it is just 1 or 2 mm more, it makes the 7x42 the most comfortable in use. I added an o-ring underneath the eyecups, which adds in comfort as well. I also have the green eyecups of the GA version and they are the most comfortable in use. However, since the aesthetics were partly a reason why I wanted to add an Habicht to my line-up, I prefer the original black eyecups. It looks so much nicer and the original black rainguard fits on it too. The Habicht is standing on a shelf in the living room, because it is an eye-catcher (at least for me) and I can grap it when I need it.
The 7x42 (and 10x40) fits my hands nicer than the smaller 8x30. The 8x30 tend to dangle a bit more too, because of the different centre of gravity. I had a hard time deciding which one I liked more: 7x42 or 8x30, but the 7x42 will get more use.
And what about the very small AFOV? I hear you asking this question. An AFOV of just 46 degrees sounds horrible and put my off at first. But in reality it isn't that bad. It is comfortable in a way that is hard to describe.
So would I recommend an Habicht for birdwatching? Not really, but an Habicht can be used for it for sure. For 10 power, I prefer binoculars with a faster/smoother focuser. The Habicht 7x42 will be my low light glass. For that it really shines.
I tried the GA green rubber variant as well. Although I liked the eyecups, I prefer the ergonomics, the weight and the looks of the leatherette versions.

