Good Morning AC. We have one of each. Corbin was 4 in March, he weighs 43-45 lbs and is tall. I have the back support booster seat, his parents have the back less one.
You can use the car seat belts with both types. Our back supported one comes with an extra strap to keep the car belts off the neck area. This one keeps them up straighter, but they can wiggle more also. At least Corbin bends forward, and gets things off the seat or can reach over to his brother. WHICH I do not like at all. So he is going back to his side of the car and use the shoulder strap again, not just the lap belt. He wanted to sit by Zane, not gonna happen anymore. He wiggles and moves way to much.
I offered ours to them when they went to TX last week, but they preferred theirs as Corbin has to have a pillow behind his head to sleep and ours won't allow that aspect comfortably. It's to straight up and down.
So that is my 2 cents on both types. Hopefully other Mama's have more information to help you make the right decision for you and your precious Girls.
God Bless and have a wonderful SONday
K. Nana of 5