I want to buy the 8 euro subscription from Netflix but I would like to know if the quality is worse than that of the premium subscription for 16 euro
Notice I hardly think a difference
HD is not included in the basic subscription.
* The tax reduction applies to Germany up to and including December 31, 2020 - it is not clear from the question whether you are from another country.
That depends on your end devices and also on your internet connection.
If you have 4K devices, large screens and a fast line, then the expensive subscription may be worthwhile.
If you only look on your mobile phone, you will hardly notice any difference between SD and 4K.