i always used big boats since Silent hunter 1.
IX-B is the best choice during the happy days until late 1941.
6 torpedoes tubes ready and 15 reloads, very good for attack convoys and
very fast surfaced, i reached 19 knts in calm waters with this sub.
More fuel, or more speed at the same range.
But remember that you have to adapt your tactics, for example donīt raid ports, german submarines rarely raid ports (scapa and what more ??). And crash dive as soon you see an aircraft, because it takes a few extra seconds to dive.
In my last patrol i sneaked to the mediterranean with my iX-B boat and survive to return to the atlantic again
(1941)
Later, you can change to a smaller VII or continues with a IX-B but operating far patrol grids , navigate at deck awash or get a snorkel and radar detectors as soon as possible to try to survive to 1944-45 .
Type VII is good in the later years for his size during scorts attacks and fast dive times. the torpedo load is reduced , but is not important in late years, because allied countermesures make attacks very difficult and the war is lost anyway.
if you want a little revenge buy a XXI boat in 1944.
17 knts dived , radar detectors in the snorkel and a lot of firepower, no more creep at 7 knots waiting the death.