I have both, play both and enjoy both.
It's a matter of personal opinions. The two games are the best (and almost the unique
) on their area.
Both games are very, very realistics when played at full realism.
SHIII is more "intuitive", not just for the gameplay, but because the WWII technology is more simple.
Maybe the greatest diference (apart from the time) is that on SHIII you "only" drive German subs and in DW you can drive subs from 3 countries, and also a missile frigate, a helo or a plane.
Ufff it's very hard to explain.
The game concept is also different becasue there had not been a modern war with modern submarines (and helos, Orions and Perrys), so DW is "limited" to small conflicts or hipotetical wars.
On the other side, SHIII inmerse you very well on the IIWW, probably the conflict which atract more people to know how, when, who, what...
As said, is a matter of what kind o play do you want. Long patrols, easy math operations for navigation and torpedoes... or technology, procesing signals from various sensors, very fast fights, radars, missiles...
Finally, SHIII has far better graphics than DW. But you can play DW windowed. And SHIII is more inmersive-claustrophobic than DW.
I will stop here because ona can speak to the end of the times and don't decide for one. Try both, and decide yourself. I decided the two
