Either bed would work for your daughter, but since you have the choice of a twin bed, I'd suggest that be the one you use for her, as the queen bed will be more useful for guests if you have anyone visit. Mattress covers for a twin bed are also less expensive. You can buy a good plastic cover that will cover the entire mattress, and zips up, at WalMart or other similar stores for about ten dollars. You might also want to invest in a plastic pad that you can put just under where she sleeps, and put a amall piece of lightweight blanket or sheeting over that, just so you will have less laundry to do on a daily basis in case of a wetting accident.
I would definitely make the change of beds for her as soon as you can. Start talking about how big she's getting and that she's ready for a 'big girl bed'. Give it a few days to a week of talking about it, maybe letting her take a nap in it, and then make the transition. Be sure the message she's getting from you is all positive regarding how big she's getting and not anything at all to do with the new baby needing her crib. Then, later on, when the baby is closer to being born... or even after the baby is born if you have a bassinet for the first few weeks... you can make it another big deal that she can give her 'old bed' that she's no longer using to her new sister or brother.