I played SH3+4 and Sub Command+DW.
The genre is TOTALLY different.
SH: I played vs AI only.
DW: I played (almost) vs human only.
SH: great focus on graphics (details, explosions), huge use of the periscope. Little use of sensors.
DW: little attention on graphics, little use of the periscope, huge use of sensors.
SH: crew management
DW: no crew management
Special features:
SH: crew management inclusive promotions and medals
DW: "multistation" - up to 4 players in the same sub (sonar;fire control;TMA;ship control).
While for SH it is vital to deliver a good visive experience, for DW it counts little (to me). The graphic detail in DW is to be found in the stations of the sub, witch allow you to gather all the knowledge you need...
Skill at sonar, TMA, fire control, evasion and general tactics makes the difference between a win and a loss.
With DW/SC i had extremely rewarding moments. With the use of the sensors, you might come up to read the mind of your opponent
(I mean, be able to have a good guess on your opponent's next move).
In SC/DW the stress situations are not rare (lot of things to do). I would say that when "into the action" things evolve more quickly than in SH.
Finally, two totally different games.
You will find that DW is much more demanding than SH in terms of knowledge requirements. The book will help you out with the basics, but it doesn't suffice by far. I find DW to be more challenging than SH (and more rewarding).
Playing multiplay DW with autoCrew ON is something you don't want to do (except auto TMA).
Little drawback: as already mentioned, not so easy to find opponents, but there are virtual navies that help out.