633 is CERTAINLY the best mix recorder, by a long shot. But there's also cost as an important factor.
If I were doing effects, interviews, or ambiance, I'd certainly go for the mix pre 6 or 3. The value is just amazing. I just think the f4 and f8 were designed with narrative production in mind, and so they work a little better for those situations. But also by going with the Zoom, you lose the trust of the Sound Devices name, which can go a long way with producers.