It seems that these two technologies becoming one and we get some animated modals.
Transitions are probably the best way to trace the state of the current, next and previous position in a visual workflow. A dummy example: closing and minimizing a window would be impossible to comprehend the difference without the appropriate animations. Same goes for flip-book, carousel and you can make thousand examples of this. You understand, right?
Gnome3 as for now uses transitions in Mutter only. This will definitely change in 3.6 that all Gnome apps will gain some “moves”. Jakub Steiner has done the first blenders in an experimental window (this is not the real Settings!).
Jakub shortly explained us the changes: “The transition introduces the modal and makes it clear which window it belongs to. The reason why we changed the modals is that they don’t currently look as appropriate with maximized windows“.
I want to add that transitions except the practical use are nice optically as long as remain low profile without distracting the user. And Gnome has succeed that.
This video was originally @http://jimmac.fedorapeople.org/gnome3/newmodals.webm but I had to upload it in YouTube to embed here.