what's the difference between...
 

aside from the obvious, why would someone choose to have other people submit galleries to their site vs. someone who uses all of their own galleries on the site?

I'm guessing the people who get others to submit their own galleries have to add a link on the gallery page which means more traffic?

Is this the only reason?

Recips would be one reason as you do get a small amount of traffic from them plus it will help with getting SE traffic.

The other is the galleries will be more unique then sponsor hosted galleries since more people would be building them.

Basically to sum it up it would be for the recips and fresh content.

Any idea what is more common? (people who have their own galleries vs. people who accept galleries from others)

The biggest sites got that way by providing FRESH content to the surfer so the surfer would bookmark and come back tomorrow to see more fresh content. If you do not already have a big traffic base, using sponsor galleries is probably not a good idea for building traffic.

Amen Sister!

DD