Some tactical models come with a pistol grip, others without. Others are offered with a pistol-grip only (no shoulder stock), but most of the tactical shotguns used by the police have a shoulder stock. (The stock helps the officer shoot more accurately, especially when shooting a target at a distance of more than 50-feet). Experts debate if it is better to use a model which incorporates a pistol grip with the stock, or a plain shoulder stock. Both are a fine choice. Some police departments use a grip model, while others do not.
In the course of providing training for both law enforcement as well as the general public, I have noticed a slight advantage to a pistol-grip equipped tactical shotgun, particularly for those who are new to shotgun shooting. The pistol grip seems help with natural aiming, especially when shooting from the hip. Further, inexperienced shooters tend to shoot high with a shotgun, and the pistol grip seems to help them naturally hold the shotgun down and on target. The pistol grip is also useful for those who want to use advanced combat techniques.