The secondary weapon shown is actually not the Ruger MP-9 or the Interdynamic MP-9 but actually the Brugger and Thomet Industries type of MP9 Pistols.
The MP9 is a compact machine pistol that is equipped as a secondary. In comparison to it's fellow automatic machine pistols (Glock 18, FMG9), the MP9 has the highest rate of fire of the three. The damage is the lowest, but it happens to be accurate, more accurate than many assault rifles.
The FMG9 is a close comparison to this weapon. The FMG9 has a much lower fire rate, but happens to have higher damage and and a little bit higher range. In terms of usage, the FMG9 is more of a longer range, stable, slow firing yet reliable SMG. In contrast, the MP9 is a ruthless weapon at close range, where combined with it's large magazine size and good accuracy, you can rack up criticals and headshots very quickly. The MP9 is probably the best secondary gun for point-blank range due to the sheer firepower of all the bullets you can fire.
The Glock 18 has a slightly lower rate of fire, slightly higher damage, lower magazine capacity, slightly higher range, and lower accuracy. Although it's also equally effective at close range, the MP9 is the only machine pistol with enough accuracy to rack up enough headshots to do some serious damage.
Recommended use with: The MP9 is a fantastic secondary when you have a long-range weapon capable of holding it's own at medium and long range. When enemies get too close for comfort, backpedal away while shooting with the MP9. Alternatively, use it to whittle down enemy health if your opponents are themselves running away, then switch to your primary when they are out of range.
Recommended use against: The MP9 is one of the few machine pistols that can deal with almost all types of enemies. For Juggernauts and Mercenaries, depending on the quality of your MP9, a single clip may not be sufficient, so if that is the case, pull out your primary and finish them off.