Its absolutley better, more accurate physics and sonar modelling, the possibility of using multistation in multiplayer scenarios.
The ability to go against other subs/air platforms/frigates whereas in the old 688i you could only fight against other 688i :rotfl
talk about variety here).
Last but not least, the amazing LWAMI mod which greatly enhances the database and doctrine files to give even more realistic setting on torpedos/ weapons in general/ etc...
Its worth going from 688i to DW.
Apart from a nostalgic sense, DW is superior to 688i in all aspects.