All things equal I side with harnessing until 6 or 7. not that he NEEDS harnessed, and there would be nothing wrong with boostering him, I just would lean towards a harness.
I clinically qualify for a benign macrocephaly diagnosis, so a big head, like bigger than at least 99% of the female population, completely off the charts, lol, and even MY head is not significantly bigger than the average persons. A slightly larger head is going to increase neck loads, and head excursion, but bar hydrocephalus or a pathological macrocephaly it wouldn't even figure into my decisions, at all. And for those conditions I would probably want to rear face and consider importing a seat if necessary
It is impossible to know what variables you would be trading up- maybe the booster has decreased neck loads, but increased head excursion. Maybe the booster is safer in a frontal impact but more dangerous in a side impact. maybe a harness is safer in your coupe but more dangerous in your SUV (or vice versa). Think about it long enough and you'll get a headache! As you know, whatever you pick use it properly, and your kid will be very, very safe.