RE: twin beds vs. double vs. bunk
I don't think the top bunk is safe for children under 6 or 7 so it may be a while till a bunkbed becomes useful. You can always set up the bunks as two twins right now and change it to bunkbeds when DD is older. If space is not an issue I would not go for a trundle. Then you can use the space under the bed for storage.
I always love the pics of little girls rooms with two white twin beds in magazines. So if could choose I would go with a bunk setup as two twins or two matching twin beds for her new room.
Michelle I
DS 1 - 7
DS 2 - 5
DD - 1