If you want to save on Skypass miles, I would say stick with Prestige on the 777, and First on the 747-8. The 747-8 First experience is more special in that you're in the nose of the plane, whereas First in the 777 is still nice but I wouldn't necessarily pay the premium over Prestige. I think the advantages of First over Prestige are a wider seat (with a door), more personalized service, and pajamas, but I have received better service in Prestige than First, so YMMV.