You should start by playing the easier human campaign first, followed by the more difficult Cybrid campaign. As you progress in the campaigns, you can recruit other pilots to join your team and have access to more advanced technology. You cannot change the level of mission difficulty but if you want a bigger challenge, either recruit a smaller team for the mission or go solo. The different types of vehicle on your team can all be individually configured. You can choose a vehicle and then outfit it with various types of weapon, armour and special equipment but a load limit will prevent you from putting the best of everything on the same vehicle.
Tanks move fast but have no shields.
The smaller Hercs are fast too, but they carry less weapons. The larger Hercs pack the biggest punch, but are easily outpaced by other types of vehicle. Choosing the right special equipment will give you an extra edge in a difficult mission. The more interesting ones include a laser targeting device for long range artillery and a shield modulator which rotates the shields to face an enemy vehicle. My personal favourite is a turbo booster which gives a temporary but big increase to a vehicle's speed.