"Die anderen Argumente finde ich nachvollziehbar, aber meiner Meinung nach wiegen sie die Vorteile des Sprays nicht auf: das ist kompakter, leichter, unauffälliger, einfacher in der Handhabung, mehr Wirkstoff"
(BTW: Sorry for the English language, my German is enough only for understanding what you wrote, not for answering them.)
So, let me debate in English
Kompakter/unaufalliger: it is true for a small 15-40 ml dose of spray but these are absolute not comparable to a gas gun. The larger 50-75 ml or larger cans are not more comfortable to carry than a small pistol or revolver.
(Fot a 75 ml dose I think you need a holster, too)
Enifacher handling: A DA-revolver is simple enough. Pull the trigger and bang. If it aint "bang", pull again.
If the spray do not eject, you have no chance.
So it is more reliable than a spray, except you carry more than one spray.
mehr wirkstoff: absolutely false. According to German laws the same maximal amount is PER dose (80 mg CS per an entire flask of spray or per a single patron).
The actual concentration of the "gaswolke" can be higher in the case of a streamer or foam spray but not in the case of a fogger or cone.
If you want to aim to the head with the (higher-range)streamers there are much less probability of the hit for first try, than with a gas guns 0,3-0,5 m wide, 2-3 m long "cloud". And you are able to easily and promptly increase the concentration with a double tap.
Make no mistake: tha gas-guns are far from being the best or even the ideal NON-LETHAL selfdefence devices, but they are better than the sprays.