Well, due to the multiple platfforms and incredible number of AI units, the scenario possibilities for DW are unlimited. You only have to visit places like
SUBSIM,
SUBGURU and
Commanders Academy & Dive Center to download many of the avalaible new Campaing and missions.
MP possibilities are unlimited while with SHIII, MP is lets say"poor and boring" after 3 games. Shall Silent Hunter III have incorporated a Destroyer Command version so you could have Sub vs Destroyer combats, the score would be completely different, but they have not and thus its MP is really poor.
About complexity, well, comparing the complexity of DW with Silent Hunter III is comparing driving a bycicle or a fighter jet.
Just take a look at the 500+ pages manual of DW and the "brochure" that comes with Silent Hunter III.
However as I said, I still like SHIII, makes me feel on the atlantic riding the waves and I like the aspect of "managing" your crew so it will be on my computer for long time.
However due to the increase use of DW among virtual Fleets because its MP possibilities, you will see after a year which game is ruling.