The other advantage with the monorail, particularly when returning to Haneda Airport, is that it is much harder to make a mistake and board and inappropriate train than it is on the Keikyu line.

Personally, I use the Keikyu line for this journey, but that’s only because Shinagawa is a more convenient station for me. On the Keikyu line it can get extremely crowded when going Southbound towards Haneda. I suspect the monorail (perhaps the fact that it costs a little more) may not be quite so crushed. The view from the monorail (which hugs the waterfront) must be better too.