I am in my first career as a S-boat capatin (S-33). I love how it gets tossed around in the waves and that it can crash dive very quickly to avoid air cover. Also you don't have to worry about those annoying stern tubes
In SH3 I started my first career in a Type II as well. I think there is a lot to be said about starting off in the "trainers"......you learn a lot of the subtletees that might be compensated by one of the bigger boats by having more torpedoes to waste or extended range and speed.
One thing I have started is to look up the boat I have been given on wikpedia to read about the patrols that it accomplished. It just gives you a flavor of what sort of work they accomplished.