IIRC this is what happened:
When my oldest grew out of his infant seat we got the alpha omega thing.
when my second grew out of the infant seat my oldest was still in the car seat so we needed a second one, We didn't want another big bulky one so we got the Britax. By the time my second grew out of the Britax my oldest was already in a booster (or maybe ready to move to a booster?) so my oldest starting using a small, backless booster and my second started using the big one (as a carseat) until about 50lbs when we converted it to a booster.
Hopefully that makes things clearer, not more complicated