It depends on the preview.
Most of my traffic is SE and gets mapped directly to the best sponsor preview, but I have found that if the previews are too brief, the surfer has not had a chance to get enthusiastic about the site and may not be ready to sign up so quickly.
Brief previews are great to quickly sort out the buyers from the freeloaders, but I'm having better conversions by sending SE traffic directly to a longer and more generous previews.
So far at least...
-Dino