I too would prefer a high backed booster. A high backed booster, especially one w/ good side impact protection is going to offer loads more protection than a backless booster. I have some great side impact crash tests of boosters if you'd like to see them. If you sit her against a wall, you can measure from the floor to her shoulders to get her torso height. It's not far fetched that she may just be too tall for a high backed booster and in that case a backless is best.
Here's a good visual of why top tethers are so important.http://i47.photobucket.com/albums/f1...uch/Tether.jpg