Keep the VII-B. The VII B actually has a slightly greater underwater range and its speed submerged is slightly higher. Think about it like this... if you had gotten a real VII-B in the war and were sucessful with it, and the boat was still fit for sea... why would you need a VII-C? The only thing is... later in the war you can't get the AA firewpower with the VII-B that you can with the C.
