I like the IX-types better, they can carry more torpedoes and that's always a plus.
If you plan your attack well, you almost never need the couple of seconds faster diving that a VII-type can provide. You also carry more fuel so you can cover a greater area.
+ YOU GET TWO STERN TUBES !!!!
you can shoot two at the pesky escort tracking you, while he turns away from the first torpedo the second will always hit !!