On the surface, for self defence the 38 Super would be the choice, now comes the "buts". A heavier pistol required to keep the recoil effect lower so you can get back on target quicker. There are far more pistols for the 9mm available so better chance to get an off the rack job that suites your needs. Can run +P+ in 9mm. If you do not handload then bullet selection is not as good in the 38Super.

In the real world I would go for the 9mm Parabellum every day.