[Pohl]

Like someone said above, using the IX is slower and heavier, altough you carry way more torpedoes the shells for her deck gun are limited to 110 only, the only good thing I find about the IX is she's the best anti-convoy sub out there until the XXI comes out (but dear god she's expensive), but only if you know what you are doing, it's to heavy to maneuver fast and it's max speed on a silent run is 2 knots, max speed submerged is 8 knots, and if you decide to go 150+m it will take good 10-15 minutes for her to go that deep, but after danger goes away you can surface and load your extra 8 torpedoes into her and try to reach the convoy again, to spend another 8 shots on it, VII is a sub for those that say, 1 torpedo 1 kill.

I recommend to get used to the VII then move into the first IX and notice the difference, best way to do so is in a multiplayer game or single mission, try both subs and see which one you like more.