I think you have been given a lot of good advice from the other moms here. I think one thing you will really want to think about here is the baby's schedule and how it would affect your other children if they were to share a room. I am the mother of three girls too (5,3, 11months), and the baby got her own room here. Babies have such different schedules from older children with their daytime naps and waking during the night. If you put the baby into either sibling's room, it could be a little difficult for them to accept that their room and their "stuff" was off limits for periods of time in the middle of the day when baby naps. The might also be awakened in the middle of the night when baby wakes up. Just some things to consider.
I also agree with what another woman wrote about possibly sacrificing the guest bedroom. If I had an extra room here, my oldest two would probably be separated because putting two girls to sleep in the same room brings a whole new challenge to bedtime ;) You could always combine children when your guests are there.
Also - an alternative to bunk beds might be a trundle bed. That would keep your toddler a little closer to the floor so she doesn't end up falling from a bigger bed as she adjusts to a "big girl bed."
As far as talking with your husband about the difference of opinion, maybe the two of you could sit down and make a list of ALL the pros and cons of each living arrangement. If you put it all on paper, it might help you both to see what would work best for your family.
Good luck with everything, and congratulations on your new little one!