Playing from 1939 to 1944 in a IXB or VIIB is not historically correct either...

All IXB and VIIB U-Boats were decommisioned in 1943.
And as far as I know the XXI Boats were actually not available for combat patrols until March or April 1945.
but to answer your question I would take the VIIB... just a personal preference because of its speed, manueverability and stealth.
__________________
E8400 @ 4.00Ghz standard vCore, Gigabyte
P35C DS3R
@ 445 Mhz, Leadtek
GTX 280,
3 x 1GB Kingston DDR2 800 @ 890Mhz, 250GB +160GB HDD, Terratec DMX 6fire 24/96,
Silverstone ST60F 600W, Logitech Z-5500 THX
