Clearly you should go with the Glock, you're familiar with them and it sounds like they're supported by your dept.
As far as the Beretta goes, I think you're really overthinking things with the slide removal scenario. If someone grabs your gun you are now in a fight for your life, even the most dexterous criminal won't be trying to disassemble it, they're wanting to shoot you with it. Perhaps you should put all that energy into practicing gun retention techniques and give some thought to back up guns and knives, regardless of which gun you choose.
Or buy the Beretta and have it fitted with a threaded barrel, if you keep the thread protector on it they'll have to unscrew it before they can remove the slide. That should slow them down enough for you to work up a plan B.
Oh yeah, FWIW I carry a FNP45 Tactical on duty and am very happy with it, they make a couple of 9s you might look at.