I doubt you'll get many responses on this forum - you'd likely get more in exchanging or on the Marriott board. You can ask the moderator to move this, but should not start another thread yourself as duplicate threads are frowned upon.
I'll give you my two cents - the CI is directly across the road from the two Marriott Desert Springs Villas, so in terms of location there is no difference. It depends primarily on what you like in terms of atmosphere. CI is small, all two story buildings arranged in several clusters around pools. Very desert-like grounds, and architecture that blends in with that. Not much else there (which is why I prefer it). The Marriotts are larger developments with tons of units, located next to a hotel with several restaurants, in a much more lush and oasis-like atmosphere.
There are lots of reviews on these and other Palm Desert resorts here available to members.
No, I wouldn't consider CI part of HGVC at all. They happen to share some inventory if you participate in particular programs, but CI topics are usually posted under Point Systems here on TUG. Since your question was about comparing Marriotts to CI I suggested moving the thread somewhere you might get more responses.
I have stayed at both places. Both resorts are very nice. I do not think you would be disappointed with either one. The CI is quiet. There were no activities that I can recall. Children and Teens might be bored. But if you are looking for a nice quiet week at a beautiful resort, CI might work for you.
DS was also beautiful. There were pools (kind of small ones though, activities, and a nice resort hotel for dinner and a spa if you wanted to do that. Lots of young kids and families were there.