The answers are in the Wiki articles.
For fighting in the Med you take the smaller boat. a slightly smaller asw target. Transit distances are short. going back to base for more torpedoes is only a few days.
For fighting in the Pacific you take the Bigger boat,the Gato. 3,000 more miles of range, longer time on patrol given the transit distances, 24 torpedoes vrs 14 for combat duration once targets are found.
If you have to buy 100 subs you can buy more than one class/type
British orderd the Amphion class for fighting in the Pacific.