I use NextSTART and WorkShelf at the same time, and I would say that even if both are basically glorified program launchers, they are quite different in how they work and nicelly complement each other. Otherwise, why would we have quick launch toolbars (or even the desktop itself) when we can always use the Start Menu (and vice-versa)? Surely there are advantages (and disavantages) in each system, that is why most people use both.
NextSTART is a text-based menu system. WorkShelf focus is on icons, not text. Personally I prefer a quick program launcher like WorkShelf for frequently used applications and NextSTART when I need to dig in for a rarelly used application.
NextSTART also provides features that WorkShelf does not (like the Startbar, hotspots, etc...) and WorkShelf does the same with its modules, etc... There are many other differences, but I will let some other Winstep user speak about them if they so wish.
