Hmmm, that's somewhat of a loaded question. IMO, stability is more often than not a combination of the OS you're running and the quality of your video drivers.
Skinnable apps tend to stress the OS in general and the OS's video capabilities in particular.
For instance, I use a W2K laptop with an ATI video card and drivers. While W2K is stable, my ATI drivers are terrible. I can't run any skinnable apps in 32 bit color mode without getting graphic corruption and (I suspect) GDI resource leaks.
In my experience, at first it's easy to blame a particular skinnable app, but it shortly becomes apparent that its the OS and drivers that are to blame as I start to have problems with all graphic-intensive apps.
Lastly, if considering comparing the functionality of each app, then you're sort of comparing apples and oranges with regard to WindowBlinds,NextStart,and DesktopX in that on the surface, yeah they're skinnable apps but each serves a different purpose. WindowBlinds skins the standard Windows and Windows Apps. DesktopX is a "freemform" object-based desktop enhancement.
NextStart (and Workshelf) is a configurable popup menu system/program launcher.
As I said, at various times I've run all three in some combination or other. The problems I have are not with the apps but with my laptop. On my desktop computer with an entirely different video card/driver, I have no such problems.
I would consider trying all three apps you've mentioned and see how they behave on your computer. You may want to verify that your video drivers are current. If you're running a Win9X/Me OS you may want to consider upgrading to Win2K as its more stable OS with better memory/resource management.
Just my two cents.