Steve,
You'll probably continue to find different accounts because there is no one right answer. The way that works is usually the right way. If you don't previously have a relationship with a publisher, then you will want to submit demos rather than worktapes. Once you establish a relationship with a publisher, he or she may prefer work tapes so they can decide on whether or not the song warrants a demo. Until that time, you do need to present your songs well. I generally recommend for people who live out-of-town to use aNashville mail in demo service. A few that come to mind that do good work for a reasonable price ($300-$400/full demo) are Jackie Cook ('The Song Cellar'

, Galen Breen ('Gator Hole Recordings'

and Jay Vern (Jay's Place).
As for guitar/vocal versus full demo, it just depends on the song. I have a couple of Sara's songs and mine on our SongU.com Jukebox that you can take a listen to (the link is
www.SongU.com/jukebox). One is called 'Arlington' which is a simple guitar/vocal that just got cut on the new American Pride CD (with songs by artists like Billy Ray Cyrus, Lee Greenwood, etc.). They also cut the song as a simple guitar/vocal also. Interestingly enough, we had a full demo of the song, but we didn't feel like it captured the emotion of the song, so we went back and did a simple guitar/vocal and that's the one that nailed it. We also have a full demo of a song on the jukebox called 'The Box' that's been on hold a few times but not cut. The songs are 24kbps streaming, so it's not the same 'quality' as the CD quality versions obviously. But you should be able to see that there's a pretty big variation in terms of what works best for the song.
Hope that helps.