I know some instructors prefer the R3 because it allows jumping in and out of different cars quickly. I use a HANS. If you go the harness and bar route you might also look into the Defnder. Initial reviews are good, supposedly very comfortable.
http://www.defnderneckbrace.com/
In any case, H&N restraints are jihad topic on Rennlist's racing forum, lots of great information there including posts from researchers.