The IXD2 is a very nice flak platform to help you get through the Bay of Biscay and to your patrol area. The dive is slow and its a huge sonar target, escorts won't have much problems finding you. Once you are free though they can tear a convoy to shreds with the torpedo load. Best haul I've ever had was on a IXD2, 201,000 tons spread over three different convoy's I encountered till running out of eels.
If you can make it to the point where you can get a snorkel moving around with the IXD2's gets easier, but you are still quickly found by those pesky planes even submerged. The VIIC's are more nimble but with a much smaller amount of firepower compared to the IXD2. I like to compare them to flight missions that the aviators would have had. With a B-17 you could be gone for 12-18 hours per mission, the P-51 might only have a few hours flying time (excluding escorting bomber groups) flying sorte's against incoming bombers so you might get 2-3 sorte's in the same time you would if you were in the bomber. Bottom line is really which better matches your style of play and what you have the most fun in
