I'm sure it is different for all families but I grew up sharing a room with my younger brother and pretty much hated it most of the time! I wanted me own space to play in and he was a bed wetter for quite a long time and he would always have to be taken to the toilet during the night which disturbed my sleep. I also have two older brothers that always shared a room and even though they always grew up very close, they used to argue a fair bit too and I used to hear them doing things to annoy each other or keep each other awake at night LOL
Like I said, it could be totally different to yours but I guess you have to think of the long term, as they it might be cute and nice when they are younger (although might be hard too when baby is waking during the night!!!) but it may get harder as they get older cuz as you get older and more independent, it is nicer to have your own space.
Me and my brothers are all very close now but it would've been awesome to have my own room when I was growing up! I was 14 when I finally got my own room but that also meant all THREE brothers then had to share. I was told I was getting my own room as 'girls need privacy as they get older' LOL
When I finally got my room at 14 it was like HEAVEN!!! I walked in to my room everyday for about 2 years thinking 'I can't believe I finally have my own room' HAHA
In your case though, you could start them off together and if it doesn't work, at least you have the extra rooms to change your mind!