I'd consider it this way;

Poser Pros.
 - You can use lights prepared to fit the pose, model etc.
 - If you need to change anything, it's in the same app.
 - Dynamic hair is only visible in Poser.
Poser Cons.
 - Slow rendering.
 - Limited to props and simplified 3d models.

Vue Pros.
 - Possible to use any 3d model - 3ds, obj, et cetera - without detail loss.
 - Built to handle huge amounts of polygons.
 - Better light simulation, plus volumetric light with dust and light gels (ex. caustics).
 - Extensive terrain tools and EcoSystems (auto-vegetation - populate a background forest in seconds).
 - Spectral2 Atmospheres (really great skies introduced in Vue 7).
Vue Cons.
 - Having to change to Poser every time you need to edit your Poser model.
 - No Dynamic hair! Boo-hoo! (I don't use it anyway, so who am I to say that ..)

Hope this helps... PM me if you need to know more :)

Cheers,

A-Spot / Thomas

EDIT: May I add to the above post that, even though Vue (in my case Vue 7 xStream) is quite resource hungry, it runs smoothly on my Acer laptop if I just enable wireframe view on the three non-perspective views (Top, Side and Front) - and that is with Auto-Update Preview enabled and Smooth Shaded in Main Camera.