Hi Alex and welcome to the forum,
No matter where you sell products in the world, your tax obligations always remain in the country where you pay your tax. So you would be under German taxation regulations for your business, no matter if you are selling in the US market or not.
Outside of the taxation and business compliance regulations of Germany, there are no other rules that would concern you.
In regards to the best market, that's something I can't advise on because it can come down to many different factors. Why not try both markets and see which one serves you best?