I find that my preference has been to use the VIIC.
As for C vs. B, I'm happy to trade off an extra knot submerged to pickup the faster diving and extra topside goodies like the schnorkel. Plus I guess I just like using it for its historical significance.
The IX was great for a new experience, just read Operation Drumbeat and you'll want to fire up a new Hardigan career in a IX. Hunting faster moving tankers in the carribean with a type IX (for me) was a totally different experience than the North Atlantic playground. The downside was TC and real-time it took to traverse the Atlantic. I had to mod my config file to allow for higher TC... I cranked it up to 4096 and it seemed to help.
I think when the new GWX comes out and Milkcows are operational I will definitely have to revisit the IX!