Real geometry vs. subsurface modifiers etc

[voon]

I am always confused about this and never really know, where the ideal line is to maintain best performance ... wether I should really subdivide my mesh and create a lot of extra vertices, or just use a subsurface mirror, and only create to-be-calculated extra verts. I'm pondering this because of displacement, where you usually need tons of verts to make displacement maps work nicely. Is there a good tutorial about these things somewhere? I work with blender, but I guess it is similar for most modellers out there.