@ bams
B.t.w I am 26 yrs old and currently doing my doctoral dissertation (PhD) over venture capital financing of e-business start-ups. I will finish next year.
I think summa summarum, you will be making almost the same amount of income at the German company as in your current job in Belgium.
If I were you, I would go with the German company. Why?
a) You mentioned that the German company is big, so I assume that you have a bigger chance of climbing the corporate ladder

Having a work expierence in Germany is surely a plus on your resume
c) Düsseldorf metro is a very nice place to have a good life, you will love it
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Is the income of about Euro 3000 sufficient for a couple?
It is more than sufficient. You guys can live like kings in Düsseldorf if you choose to - you can go out as often as you want in the Altstadt/Königsallee, you can pay for any exotic holiday that you choose, you can buy virtually everything, you can even have a kid, you will not have any problem financing the kid
It all depends on the kind of lifestyle that you have. Do you want to live close to downtown or better far awy from downtown?
Düsseldorf is not that much expensive than people think - you can rent a flat in Düsseldorf-Oberkassel, Düsseldorf-Zoo, Düsseldorf-Messe, Düsseldorf-Rath or even in the adjacent sub-urban towns of Mettmann (10 minutes from Düsseldorf) and Hilden (10 minutes from Düsseldorf). I myself prefer Hilden and Mettmann better because it is sub-urban, more green, more educated people, nice big gardens and lower rent. Let say if you guys rent a flat for Euro 800 warm, you will still have like Euro 2000 at your monthly disposable.