Well it's even more complicated because you may want to wait for te IXC for even more range. See below thread for more on that.
http://www.subsim.com/phpBB/viewtopic.php?t=47241
The IXs have more torpedoes, and if you like going to far off places hunting then the IX is a good boat. Some people prefer the VII because it is more manueverable- I've been reading that often the IXs were considered too sluggish for convoy attacks ( atlerast from '42 on)
I think I'm going to start a 2nd career, so I have one with long range boats and one where I stick with the VIIs. Even if you stay with the VII the VIIC can carry more AA guns.
You may want to upgrade & try the IX for a while then if you don't like it just go back to a previous saved game.
The IXs are nice for getting to the easy hunting off the US coast when they enter the war, but harder to survive in later years because of the longer dive time and beeing a bigger, less nimble target.
Personaloly I think I'm going to try to get a IXC just for something different and because U-505 is located nearby.
Well, hope I've added to your confusion.
