C.B.
I say let them have their own space too! It is a huge gap in age, and I know my 3.5 year old really needs his own space, and his place to go when he needs to be by himself. He's able to play in his room, it's something that is all his, and something he takes pride in.
If they each have their own room, then it gives them some place to take the other, or to be away from the other.