I usually mention both. The short-version spiel goes something like, "You know, most people are familiar with the recommendation to turn kids around at a year, but that's actually a bit outdated. Current recommendation is to rearface for at least two years, or better yet until they max out their seat. Now, based on everything I've seen/heard/read, I feel so strongly about the benefits of rearfacing that my kids sit backwards until they're much older... usually four or so." They usually have a response at this point, even if it's "Wow!" in a tone that says, "you're crazy!", lol. Then I continue with "I think a lot of people think that recommendation is nuts, I know I was like "ummm.. no thanks," when I first heard it but here's why we suggest it -- blah blah blah, 1/2" vs 2", blah blah, internal decapitation, blah blah." Response: look of horror and seat goes in RF.
I think, as pp have mentioned, that if you throw 4-6 out there as an initial goal people will write you off immediately. I find that I have a lot of success by conceding the point that it's very different, and sounds slightly nuts, which gets parents to listen long enough to get to the WHY of the rec. It's been highly effective for me, though it may not work for everyone.