Often the toddler sizes have extra room for heavier toddlers. A little girl who is normal size should be able to wear either sizes but with a thin one I would skip the toddler sizes now and go straight to the girls wear department. My girl went from 3T to 4 slims in the girls department for jeans but they were a bit longer. So we rolled them up the first 6 months. Then the sizes fit her all the time. Jeans we go to the girls department.
The only thing I would say is that the baby/toddler department clothes are usually cheaper so when it comes to summer shorts and tees, some cute little dresses or outfits, we always stay in the toddler department as long as possible. I can easily see my self spending $3.95 on a pair of Garanimals but not $12.95 for a pair of girls "cute" shorts.