The derby double is upgradable. Go with that if your sold on a 2 shooter. If you can afford it get the four shooter. Even if you have only one dog you will at some point wish you had gotten the larger model.
Would there be any disadvantage to getting an eight shooter instead of a four shooter? I don't have a bumper boy yet, but for a hundred or so dollars more, I was thinking one day I might get an eight shooter, or maybe a twelve shooter. I imagine they are a little heavier but that would keep them from bouncing out of the truck.
8 and 12 shooter BB's are best reserved for those with lots of dogs.
They both are bulky even for a man, and too much for a woman. You will need to drive into the field to set them up or use utility vechicles such as mules or large ATV's to move the equipment around. They are tough and slow to drag, even on wheels, over rough terrain.
The 8 shooter has a shoulder strap on it and isn't that much more bulky than the 4 shooter. It is the same height, just a little heavier. The 12 shooter has wheels. If you like the bi-directional option, only the doubles and 4 shooters have this.
An 8-12 would depend on how many dogs you have. Lets say you have two. that would give you either 4 of the same mark with the 8 and 6 of the same mark for each dog with the 12. I don;t re-run marks unless there is a good reason. If you have 2 dogs and a 4 shooter you can run bi-directional marks so each dog would get two different marks (having one 4 shooter), or if you dont use tha bi directional you have room for one no bird per dog.
If you have more than 2 or 3 dogs you might want an 8 shooter.