I think quality is important, no point in having something big if the quality is @#$% . I just found that the monitor colour and sharpness is so much better than the laptop (and I thought it was magnificent when I got it lol) ...The size gives you so much more visual information too ... it certainly makes life a lot easier.
The Dell monitor has a really good rap too. Some good advice I think was given here, get the very best you can afford
I have mine set up as dual monitors, all processing happens on the big fella and all the mundane stuff happens on the lappie.
Christian that sounds like a nice way of working, I might have to give it a go.