Welcome! We do not have data telling us definitively that either a harness or booster is safer for children who are at least 4 years old, at least 40 pounds, and are capable of using a booster correctly. For a 5.5 year old that is 42 pounds, either is very safe when used correctly. Before making the jump to a booster, however, you will want to be sure that she understands that she is now responsible for her own safety and needs to sit still in the car and not mess with the seat belt. A few kids her age are mature enough to take this to heart and follow these instructions, but many will take their new "freedom" as a chance to wiggle, reach for toys on the floor, etc. Also if she sleeps in the car you'll want to check that she stays positioned correctly while sleeping.
If you do go with a booster, test it in the car and make sure it positions the lap belt down low on her lap, touching her thighs and off her stomach, and the shoulder belt across the center of her shoulder - not falling off the outside of her shoulder or cutting into her neck.
If you go with a harness make sure that you check your car and car seat LATCH weight limits to see whether you can use lower anchors to install the seat or need to switch to a seat belt installation.
If she hasn't yet outgrown her current seat then there is no harm in continuing to use it until she is booster-ready (or it is outgrown).