Take it from this old VIIB kaleun, I've had many successful patrols in my boat and I was faced with just this exact question you're having now.
You can hold "upgrading" to the VIIC until the start of 1942, if you want to put it off for that long. The extras that you can get with the VIIC, such as FuMb, anti-sonar coating, and the snorkel, will make life a little easier as the enemy gains better technology and tactics to make your chances of going to a watery grave very high indeed.
Don't worry too much about flak gun upgrades persuading your choice. The damage a single airplane can inflict on your u-boat means you're much better off diving to avoid them, rather than getting into a gun duel. This is even more true, later in the war, when their radar can fix your position and then have enemy Hunter-Killer forces swarming all over you with depth charges.
On the other hand, you could also jump to a larger class and go with the seekuh type IX u-boats. They do tend to wallow in the waves like a sea cow, hence their nickname, but they're the best boats for patrols far abroad, such as sinking merchants off the American coastline and into the Caribbean.
A VIIC will get you there, but with barely enough fuel to do much more than strike a target or two before having to turn round and head to base, arriving home on fumes.
So, your choice of boat really depends on what type of patrols you plan to do in the future but your VIIB ought to be fine until '42. Then again, you might even be the sort who would enjoy sticking with what you got until the end.
There's no hard rule that says you must change u-boats or upgrade anything. If you're having fun, that's all that matters.
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Still sailing the high seas, hunting convoys with those who join me.
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