Hi there.
A high back booster helps position the shoulder belt, give her someplace to rest her head when she's tired, and has shell around her to remind her to hold still. In a frontal crash, we have no evidence that it's better to be in a high back versus backless. And there are no side impact standards, but she's too short to benefit from most side curtain airbags in just a backless booster right now.
The belt guides for the Symphony are above her shoulders? Most six year olds are getting close to outgrowing the seat that way.
We're happy to help you with high back or backless recommendations if she's outgrowing her Symphony.
Wendy