sharing a room

I have an 18 months old boy and a little girl on her way. they are going to be 2 years apart. we're trying to figure out whether or not they should share a room (once the baby is at least 6 months old and is a better sleeper). we have another room so they don't have to space wise. but I'm trying to figure out what are some advantages and disadvantages of each option. any advice?