As someone else mentioned, the taxes are different.
Namely: Salary is taxed lower than dividends. So the German tax authorities checks very carefully that you don't pay salary instead of dividends. If they determine that you paid out dividends as a salary, then you'll be charged with tax fraud.
Now you might say, "I don't care about paying a bit extra in taxes, so I'll pay it as dividends as they wish"
The problem is that you can only pay dividends the year after you earn the money.
If you can set a fixed salary which you can keep paying throughout, and then wait for the dividend payments next year, that's fine.
But what if you want to pay yourself wildly different amounts of money each month based on how much you managed to charge your customers? You can't just keep adjusting your salary up and down every month with a corporation.
So here's where something like a sole proprietorship may be simpler from that aspect?
Another thing you want to look at is "how easy will it be to dissolve the operations?" With a GmbH/UG it takes several years and potentially many thousands of euros in accounting fees. Not sure about the foreign corps. I think German sole proprietorships are simpler in either case?
Also, Germany has a "Moving away tax" where you get taxed on the fictious value of your company if you move away from Germany. This fictious value can be quite a lot more than what you'd actually get if selling the company.
Yet another thing: Depending on your setup, you may be covered by different rules regarding health insurance and pensions. If you don't make a lot of money in the beginning, it may be best to stay in the government insurance. But if you think you'll make a lot of money, it can be better to be able to do private insurance instead? There are rules on how you can move back and forth between government/private, so this is another area to consider carefully.
This is my understanding as a layman, please check this with a competent local tax expert before acting on any advice here.
