I don't have this problem as well, but two of my friends that have 3 kids, put both the older kids together and the baby in its own room. And for both worked out fine.
You never know, your son might change his mind once the baby comes and really want his little brother in the same room.
I plan on the baby sleeping in my bedroom until he/she is sleeping atleast mostly through the night. I will breast feed and I will have the little one close for a while. But for naps during the day, etc. I will probably use the crib after the first 4-6 weeks.
I would put the 2 older kids together till the baby slept throught he night or even till they were in a toddler bed. It is not fair to the older children to have a baby in there rooms that wakes up several times a night. It might disturb their sleep. Once the baby is a toddler they can share a room with the child of the same sex...That is what i would do
I have never had to deal with this yet. BUT if it were me, I would probably want the baby with the more mature child. She knows more and plus if your son is jealous, he may not take it so well with the new baby in his room. This is a tough one though! I'm not sure what I will be doing once I have this baby. My son is 2 1/2 and he and his little brother were just 16 months apart and he always adored his little brother and was never jealous. But he is no longer with us, so I am not quite sure how he is going to react to this baby. He seems excited, so hopefully it will work out and they can eventually share a room (if I have a boy)