Fleet boats for me! I just started playing last month, and I've learned more about the history of the US Submarine force since then than I did during 9 years in the Navy (including 4 on a real sub!)
Especially since it inspired me to start reading up on it. Right now I'm reading
Submarine! by Edward L. Beach. Awesome book. The game is more enjoyable because of the book, and the book is more enjoyable because of the game!
I also just finished reading
The Terrible Hours. Again... very incredible stuff. I had no idea that Fleet Subs had so many exciting stories.
I'm sure I'll eventually go for SH5... but that will probably be WAAAY down the road when I've finally sunk every Jap vessel in the library.