We have the Britax Duo Plus at the moment for DD and although we're happy with it, she's quite low down in the car meaning that she has to crane her neck to see out of the window - and usually ends up just falling asleep. Is there one where she'll be sitting higher up without us just putting a small cushion under her?
You should n't use a cushion , it will limit the effectiveness of the seat in an accident. I think you'll find most seats are a similar height at that stage, presumably you have low set seats in the back of the car. Bear in mind that a high back booster one (next stage up) is even lower so you may want to reevaluate the car itself when it is time to change.