Hi J.,
Pros and cons: Twin bed will allow less bulk to launder - more economical, environmental, and takes up less room in the bedroom. Full bed - you can lay down next to a scared or ill child when needed with the additional room.
I had a full bed frame and full futon couch/bed (from my single days) so I used them with my children instead of replacing them. The bed is a slat bed and the futon is similar to a slat bed in that both have slats going across the width of the mattress which eliminates the need for a boxspring. Boxsprings tend to wear out before mattresses. I have a foam mattress. I find it MUCH more comfortable to sleep on the slat beds with their spring mattresses than on my foam mattress with it's lousy boxspring. I'm highly interested in replacing my bed frame with a slat bed and getting rid of the boxspring.
I used rails to keep my sons from falling out of their full size beds. Even though the beds are roomier, children still fall out of them.
So twin size would be the most economical. A slat bed or futon bed is most comfortable, you don't need to pay for a boxspring, and you don't have to dispose of a boxspring when it fails. I think slats supporting the mattress allow the mattress to stay in better shape for much longer.
Hope that helps,
: ) Maureen