J.☯.
My son is 3 1/2 and is currently in the Evenflow high back booster with the 5-pt harness. He is about 40 lbs now, but definitely not ready for a booster. I'm way too nervous I would suddenly look to the side and see him smiling at me after unbuckling himself!
The carseat he is in was not much more expensive than a highback booster seat and I wanted him in the 5 pt harness, so it was worth it to me. I'm pretty sure it's also a lot less expensive than the Graco Nautilus (or similar).
He's very safe, so that would be the choice I would make. It really is nice having carseats in both cars.