Personally, from a purely aesthetics point of view, I find the VII's much more appealing then the IX's. To me they just look like "the" iconic submarine, which I guess they are in reality.
Of coase their quick dive time and maneuverability come in pretty handy as well, but I prefer the VII's just for their looks, call me superficial.
Edit: Im curious why the VIIB would be considered a better hunter then the C, arent they virtually the same, other then 0.5 knot speed difference?