Well, hard decision.
Personally, I like the VII more than the IX. The faster dive times of the VII saved my live maybe more often than I could count. On the other hand, long travels around Africa or to the US west coast are a little bit too long in my opinion. Nothing is more frustrating travelling half around the world only to get killed by an airplane
But, like already said, it is a questions of tactics (well, or personal favor) which boat to take. If you like long hauls, planning long-termed attacks on a convoy or simply seeing the world, the IX is yours. Otherwise, I think the VII will suit much better.