Personally, I ran my first career as a convoy predator (sorry, dumb term), so I fully upgraded to a VIIC/40 before getting deep-sixed in March of '44 by an American HK group (I gave 'em hell! Sank two of the escorts and damaged the jeep carrier before they got me).
Now, on my second career, I started off with a VIIB in '39, then switched over to a IXB as soon as it became available, and right now, I'm in December of '43 with a IXC (currently patrolling the Eastern Seaboard, which can be fun).
If you take a VII series boat, you'll be attacking a lot of convoys in the North Sea etc. On the other hand, if you take a IX series boat, you'll be going anywhere from the Eastern Seaboard to the Carribbean to the South Atlantic (that patrol was boring) to the Arctic (grid AK something or another...south of Greenland and north of Iceland)!
Pig boats rule the world!
lol