In regards to practical sets Ridley has stated that it is better to build it, light it and shoot it by preference. I think he quotes Trumbull as guiding him in that and his views don't appear to have changed over more than thirty years. He likes to operate the camera on occasion so I reckon sets are as important to him as they are to actors. On A L I E N he made the point that switches and dials on the Nostromo bridge actually did things and that all helped the actors feel the reality of the situation.
I've never really got the impression Ridley would be a director to rely too heavily on green screen, thankfully he's a film maker that regards CGI as one of the tools of the trade and will use it to augment reality. Obviously Prometheus will probably be his most digitally enhanced film but he's a very artistic director and he doesn't want effects to jar with the gritty reality of what he has shot practically. I can't, off hand, think of any Scott film in which CGI has looked as obvious as in most recent blockbuster films. As to the issue of cost, again Ridley has mentioned that it is no cheaper to use quality CGI than it is to do things practically.
To me it one of the big selling points of Prometheus, that it is an epic film, made in the tradition of classics, but with the advantage of digital enhancements under the guidance of a director who is a genuine artist. I agree with Cameron's point that it's about time Scott got back to Sci-Fi where he can really spread his wings creatively. That Scott is doing it from a 'practical' starting point is wonderful IMHO.