Not a frequencer to the morrowind CS forums, but it should still be the same info.
They are different programs, nifskope allows you to edit and view meshes (its not actually a modelling program); such as changing textrue paths and allowing you to add parts from a mesh (as long as the parts are seperated), or copy parts from one mesh to another. Blender is an actual modelling program, you can change the actual vertices that make up the shape. Ig you need to edit a model nifskope may be able ot do it, but blender will be able ot do it.
if you're making new meshes....then you'll have to use blender.