My gut feeling is based on my personal experience in car accidents.
I have been in three accidents. One serious rear-ender, one minor rear-ender (I was the one being rear-ended both times), and one accident where we vaulted end-over-end down a steep embankment at least twice before landing on the roof of the car.
I know they say frontal collisions are the most common, but so far I'm 0-3 on frontal collisions.
Neither of my rear-ending accidents, even the one that SMASHED my trunk (I was hit in my honda civic on the freeway by a 1-ton Chevy work truck; hardly a fair fight) were the type to cause life-altering injuries to anybody involved. I was hurt, but nothing was broken and I recovered fully eventually. I doubt that harness vs. booster vs. seat belt alone would have made any difference to any passengers as long as what they were using fit them appropriately.
The end-over-end accident, however, was pretty serious. Two of the three adults in the car were seriously injured in our regular 3-point seat belts. Nothing life-threatening, but both of us that had injuries still suffer after-effects from them today, and it's been over 17 years now. My infant son, buckled snugly in his infant bucket with a 3-point t-shield was uninjured (then again, so was my now-ex-husband who suffered nothing worse than a broken pair of glasses).
I have a hard time imagining how a 5 year old would be as well protected in a booster compared to a 5-point harness in that accident. Luck plays a big part in it, but so do restraints. NASCAR drivers wear 5-point harnesses for a reason. And yes, I'm fully aware that they have other safety devices as well, but the 5-point harness is part of that. And they were using 5-point harnesses for years before anybody came out with a HANS device to reduce spinal injuries, and most of the time they worked pretty damn well in a lot of very spectacular crashes.
The only time I think a regular 3-point seat belt might be superior to a 5-point harness is during pregnancy. I feel a lot more confident about a 5-point harness based on my experience and logic. I know we don't have data proving it, but we don't have data to contradict my gut feeling, either. So in the absence of real, decisive evidence to show that a booster is safer than a properly used harness system, I will choose a harness system for my child whenever possible.