In theory, the .40 may be better. But, assuming I could get a .40 SMG, I would still take the 9mm. It would be more controllable and more able to put multiple shots into the same area at short range, which is the rationale for an SMG.
Plus, the cost of new guns. And 9mm ammo is cheap; .40 is expensive. Could either of us explain to a city council that knows guns from beans and that is about all, why the difference is worth the cost? And why we need "super powerful bullets" that can destroy whole buildings (remember the anti-gun folks always show up)? And why what is good enough for the Armed Forces isn't good enough for the Blotzville police?
I think I would take the 9mm and spend the money for training to (hopefully) keep the more trigger happy troops from blowing away too many tax-paying citizens.