The type IX vs type VII debate is as old as the hills! Both have strengths the other does not have.
example
VII:
harder to detect, smaller profile
faster dives in an emergency situation saving the boat from damage.
deeper diving
better underwater range
harder to hit when attacked by aircraft.
IX:
Much more fuel, more possibilities
larger payload of torpedoes
more powerful deck gun
an extra aft tube
top speed surfaced.
the type IX is intended to be a heavy hitter, its a plodder basically, and it can enable you to potentially sink more targets.
The type VII is more of a fast attack, get in fire it off, get out. Much more suited to convoy operations than the type IX is.
I personally prefer the type IX's for the extra fuel and weapons. If you use them right they can be devastating, but for gods sake don't jump into your type IXC and expect VIIC performances because you won't get it.
Type VII's are better in terms of submerged performance, a good balance of firepower and stealth, type IX's tend to lend themselves more to firepower than stealth in the grander scheme of things. But that does not mean they are not sneaky, I've attacked convoys in my IXC on the surface, and got away on the surface, they can be stealthy in the right circumstances.
They can't dive as deep or stay down as long as a VII can but its up to you, if you want a boat that can dive like a monster, and better at evading escorts the VII is your boat, if you want raw firepower and a "go anywhere" feel, the IX is for you.