Lower ones generally recharge very quickly higher capacity shields generally have lower rates of recharging. You generally choose between lower and higher capacity shields.
The ships are: Escort = Tactical Officer, Cruiser = Engineering Officer, and Science Vessel = Science Officer. Ship types are meant for certain careers, and you will get bonuses if you choose the ship in that career. Cannons can only be mounted in front slots. Any class can mount lesser cannon weapons. Commander(10-19) = Tier 2 Vessel where you must select between Cruiser, Escort or Science Classes (Hint: Many players seem choose the firepower of the Escort Class which exclusively mounts Dual and Dual Heavy Cannons). As you progress, the upper floor of the Shipyard will be used to access subsequent ships and ship equipment which are earned by rank: The Shipyard is where the outfitting, etc.
Once you are commissioned to "captain" a starship at the beginning rank of Lieutenant, you are sent to Earth Dock to collect, outfit, name and modify your vessel.