Well, I've only done super8. As much as you want to argue about it, super8 is shot mostly for effect these days. The grain is very visible, and this is a positive. You can shoot feature films on 16mm. It's expensive, though, but much less so than 35 or 70mm. I don't know what you mean by straight-8. The two other formats are double-8/regular-8 which is 99.9% outdated by super8 or single 8 which is a wonderful format, the Fuji system, which unfortunately is only produced in a limited amount of film stocks. It allows full rewind allowing you to film in reverse(!) for more than 5 seconds and uses an in camera pressure plate for more stable images.
Super 8 is great for learning film and playing around, plus shooting for effect (try removing the film gate and filing it along the side opposite the sprocket hole. With proper telecine, you can acheive close to 16x9 aspect ratio.). If you seriously want to film something, 16mm will work but is much more expensive.