T.F.
How old is she and how tall is she?
When my son was 5 years old (he's 8.5 now) my friends told me about the importance of 5-point-harness car seats. So I bought the Britax REGENT. It's huge and he recently outgrew it. But it has a cozy back and his head was always supported by the sides. I highly recommend it.
I have a 4.5 yr old daughter and I would never put her in a backless booster. It's not safe enough for her age and weight.
She is now in the 5-point Graco Nautilus. (The Regent seems too big right now.) And when she outgrows that, she'll be in the Regent.
Sunshine Monterey is a booster with a back (and great head/side support for sleepy kids) but it's not a 5-point. I was hoping my son would agree to that, but he doesn't want it.
He is now (age 8.5) in a backless booster. He says he prefers it.