Hermann,
I have three 8x30 (one newish military, one newish rubber armour, one mint older) and two 10x40 (one new rubber armour, one mint older). I also have two older 7x42 and a 6x30. I'm adding more constantly.
Your question is a tough one. At their price, Get both soon, and get them new if you can, the 10x40 in rubber. They both have a much much larger apparent field of view than the 7x42. Don't muck about. I have no idea how long Swarovski will continue to make these for. They have a 35yr warranty and Swarovski will keep a full backlog of parts after production ceases. The 8x30 is reputed to be the sweet spot of habicht, and it is truly amazing, but what shocked me was just how good and sweet the 10x40 is. It is stunning. Monstrous resolving power. In fact, if I was to have only two binos it would be these two (I have a full set of alphas including 32 and 42 SV). To me, in some ways the 10x40 Habicht has an easier view than th 8x30. It has the same brutally bright and razor sharp 3D image. I've looked for it, and looked hard, but I see no down side optically whatsoever in the 10x40...just a bit more power with a slightly larger and less fussy exit pupil. Glare resistance might be slightly better in the 10x40 too....but a little glare now and then is something I take just fine.
I'm also a big fan of their old school optical formula....a big fat sweet spot with slight degradation increasing nearing the edges. No rolling ball...no strange compensatory optical phenomena to ensure a sharp to the edge image which I never nauseatingly swivel my eyeballs to use anyway...on any of my binocs. The big sweet spot is what matters most.
The newly coated habichts are so bright, that in brighter conditions, the vintage ones with their yellow tint can sometimes come as a relief.
If I was put on a desert island with one bino to last my days, I would probably choose the 10x40 Habicht in Rubber Armour. It's still very compact. Due to size, The Bino I have had constantly in my man bag is the 8x30, but lately I've been carrying the surprisingly slender 10x40 as well. The 10x40 may replace it for a while as my daily carry.
These habichts take a bit of technique, practice and skill to use...You don't just shove them into your eye sockets for instant gratification like you can with a Swarovision. But To my eyes, i do believe they are optically superior. 3D, depth of field, brightness and sharpness, value and robustness...and that very rare x-factor 'optical pop'.
Ok... My thoughts,...Given you have the 7x42 (I assume) get the 10x40 first, then the 8x30. Don't forget that the rubber armour gives you slightly better eye cups, and it won't slide about in a car etc.
Please feel free to ask me any more specific questions.
Rathaus