The common theme here is that game allows a number of variations in game play as pertained to your boat.
It's always a good move starting off your career with a VIIB with nearly 3 times the eels as the class II you can't really go wrong especially since you start in '39.
After your initial boat it's really a rock paper scissors choice....more eels or quicker dive time....that is VIIB/C or IXB/C.
Personally I held onto my VIIB for a while until I could afford the IXB. I abused the loadouts of the IXB until '42 when I discovered I needed a agile boat under water as well as diver. I simply didn't want to take the DC's in the chin.
I rode out the VIIC and picked each and every upgrade possible and still was able to cash in and get the XXI, which to my disappointment was a little too buggy for me, the lack of recharge drove me nuts
