When I was pregnant with my second, we moved my daughter into a full size bed at 18 months. She slept better actually, but she wasn't in love with her crib. I am due in two weeks with my third, and we moved my second into his big boy, full size bed back in October. He liked the crib much better than my daughter, but he still sleeps well in his bed. Now, with him I wasn't pushing anything, because I knew I was going to be keeping my newborn in the bassinet in our room for a while, but my son actually wanted to sleep in his new bed, full time, about a week after it had been set up. I don't think that your daughter is too young to be moved, but there will be a lot of changes in her life once your son comes, and her sleep may be broken by the baby anyway. My thing is why try to move them in the 3-4 months that you have with the baby in the bassinet, during which you are going to be up with a newborn anyway. If you move your daughter now, and let her get used to the new bed, at least you have a chance that she will like it quickly, and sleep well by the time the baby is born, and then you won't have two children up during the night. Plus, honestly, in a few months, your sleep will be broken by all the potty trips in the middle of the night anyway :).
Good luck and congratulations!
J.