Yes a 5 point harness car seat is always gonna be much safer than any booster seat.
The 5 point harness will provide more protection and hold the child in the car seat and prevent them from flying forward etc in a crash and the 5 point harness will keep the child safely secured inside the vehicle in the event of a rollover.
Seat belts do protect some but they can still sometimes allow a child to be ejected from the vehicle especially if the vehicles seat belt doesn't fit the child properly.
If the child is still able to fit in the 5 point harness car seat then they should remain in the 5 point harness car seat as long as possible before moving the child to a booster seat.
I would recommend going with a high back booster car seat because the high back helps to protect the child more in the event of a crash.