Hard to choose but I'll go with the type IX. (b or c doesn't really make that much of a difference) I like the VII for it's manoeuvreability and faster diving.
But the IX has a lot more torpedoes. And you can sail her to the Caribbean! Those two combined offer the possibility of multiple-month patrols
And the IX looks good. (The VII too, but the IX just slightly better IMO)
So I'll vote type IX, but I love (and use) them both.