As a company or association and as a freelancer, you should always worry about your appearance. It should be as professional as possible. So you should always use for your stationery Korespondenzen. You should also consider having your stationery printed professionally.
Whether you print your own stationery or have it printed, use better paper. Most people use cheap copy paper (80 g/m²). For a professional appearance, this is not really suitable. Experiment with different papers and try them out to make sure that you select a paper on which the ink runs from inkjet printing. Consider a slightly thicker paper such as 90 g/m². Also ensure that with the weight of the paper you do not exceed the normal weight limit for standard letters. Otherwise you will have to pay more postage.
Nowadays, there are different beautiful (white) papers made from 100% recycled paper - so that you can actively contribute to environmental protection. Should you not like a pure recycling paper, you can instead use an FSC-certified paper. The abbreviation FSC stands for Forest Stewardship Council. Resources for paper with this label come from sustainable forests.
If you have more stationery printed professionally, you should think about, which information will stay and which will be dated after a while. For example it makes little sense to have an address printed that will be changed soon anyways.
Even if it is cheaper to print more, do not print too many. Otherwise, years later you will have stationery that you do not really like anymore.