It's a case of where you want to hunt, and for how long (if you are using realistic fuel consumption).
But, in the later years, it comes down to survivability.
In my current career I started with an IXB - more torpedoes, and a 10cm gun packs a bigger punch than the 8.8 gun on the VII's.
It's ideal for early war when escorts aren't at their best, have no sonar, and the merchants are unarmed.
However, as the war progresses things get tough in the big boat.
Mid war I have taken to shooting the front four tubes, two torpedoes per target in a convoy, from about 2000 m and then immediately proceeding deep and directly away at silent running.
Take your two good merchants and then you can find another convoy some other time.
If you want to gun sling in the mid war years, you'll need a VII boat to survive convoy attacks.
It's mostly to do with your particular attack style, the locations you want to hunt, and how far along in the war you are.
Good luck.
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