The decisive factor is the condition of the walls. If they are plastered cleanly, you can paint them without any problems. If they have small cracks, unevenness or several drill holes in them, it is best to cover the blemishes with wallpaper.
If the walls are poorly insulated, as is sometimes the case in old buildings, you should paint them, as wallpaper can promote the formation of mould. In brightly sounding new buildings, on the other hand, suitable wallpapers have a sound-absorbing effect. Advantageous is also their heat-insulating property.
A further factor is the design scope. While with wallpaper you only have the choice between different prefabricated colours, motifs and patterns, you can be completely creative when painting. You can mix your own individual colour, create a variety of patterns using different techniques or, with a little talent, create your very own work of art on the wall.
Of course, the cost and time involved are also important factors in the decision. A bucket of paint, a paint roller or spray gun and the right brushes tend to be cheaper than several sheets of wallpaper, paste and accessories. If you're an experienced DIYer, wallpapering is usually quicker. However, if you regularly change the look of your four walls, repainting is much less effort than a literal change of wallpaper.