Granted these type Derringers have a lower capacity, but they can have larger caliber than 9mm. One model can selectively fire:
45 Colt/.410 Shot Shell(rifled)
.357 MAG/.38 Spl
.45 ACP
.45 Colt, Only
.45 Glock Auto
.44 Special
44-40 Winchester
.40 S&W
So the question becomes: more bullets, or fewer but bigger ones?
For me it is easy, a pistol that delivers 7, 9mm at less weight than a 2 shot derringer.
A situation where I am afraid to engage with a 9mm semi-automatic, is usually unlikely to be improved by having a 2 shot derringer instead.