Hi,
up to v.2005 there was still a big difference btw the two, to summarize it roughly DesignStar was the inheritor of Cosmos/M non-linear while CW was the structural linear part of it put inside a totally different and new (integrated) interface.
But it was no mystery that the plans were to progressively put inside CW all the capabilities of DesignStar (Cosmos/M) and probably GeoStar (whose interface, not very dissimilar to the old Femap if I remember well, has been starved to death back in 2002 or so...). In fact, as far as I know from undirect sources (I don't use Cosmos any more), the latest version of CW has material & geometrical non-linear capabilities, as well as extended contact capabilities, cyclic-symmetry handling, and the possibility to handle mixed models (beams + shells or solids).
Hope this helps...
Regards