Type IX boats are good if
- you want more tonnage at the end of a patrol.
- you want a little more toughness for duking it out on the surface.
- you want a slightly better punch with it's 108mm deck gun.
- you want to kill the incomming litter of catalinas.
- longer range etc.
Drawbacks:
- Detection! You will be detected much easier once they find out you're nearby. Prepare for withstanding long periods of depth charging at near realtime TC.
- It's not a VIIC
Suggestions:
- Switch your strategy and tactics (duh!) if you were used to a type VII. Strike from longer ranges and keep in mind your boat is much bigger and near impossible to hide in scenarios where a clever type VII kaleun would confidently manage to escape.
The Type VII:
Ah what a boat. This is the choice of an artist, not a tonnage hording bonehead. The type VIIC is a far more capable boat once upgraded.
- Medium sized U-boat. Comfy interiors, solid wood panelling, no mails, no telephones. All in all, a smart seawolf unit that's much more effective even though it does not carry all the eels a type IX does. This boat can go places where a type IX simply won't fit!
- Survivability! You can escape all the nasty ASW cheap tricks the tommy's are employing these days. It's much easier to hide your flanks due to the size of your boat. Later you can get a type VIIC/41 boat that has a much thicker pressure hull and you can go way deeper than a type IX can even dream of.
- Due to the advantage mentioned above, your tactics have precise results each time that will give you confidence.
- This boat will save your time by enabling you to escape faster and better and thus return to higher TC and onto the next encounter and victim(s) much faster.
- Your bridge is a familiar, roomy place and with a type VII you feel "at home" if you also happen to like the Das Boot movie.
Disadvantage:
- As I said, if you want more tonnage with little skill, you'll need more eels that only a type IX can provide.
- Don't even think of staying surface a moment longer when an aircraft is spotted or detected. No heavy flak and some fragility means you should avoid them at all costs, even the fat, slow moving "churchill" types.