Gotta agree with STEED on globetrotting
value of IX type boats. But trying to attack an escorted convoy in one of those is a suicidal tendency. VIIb on the other hand is the most agile of them all (except XXI) and historically the most successful boat of the whole war.
Even though i like my VIIb the most i'm quite eager to try the "big kahuna" and sail it to the tropics.
Cheerio.