Hi Dave
My Seven (with clams!) was fitted with interia belts only when i bought it. I had a set on Willans on order (BC bulk buy) at the time of purchase as I too just didn't feel secure using interias. Car now has both belt arrangements incidentally. Quite a few of us showed interest in the Willans in particular so Luke Beaumont arranged the bulk buy - top fella
Putting aside 4 or 6 point, the main differences are belt width, release mechanism and adjusters.
Belt widths - usually 2 or 3 inches, think the most popular is 3" shoulder and 2" lap so you get good width across the shoulder and chest to spread the pressure and 2" on the lap as they are more comfortable and don't pinch so much.
Release mechanism - aircraft turnbuckle is the one to have.
Adjusters - go for the ali adjusters is you are happy to go for the extra expense, much easier to loosen the straps than the steel ones, though the steel ones are perfectly liveable with once used a few times.
One other thing to note is that the lap adjusters for the Willans are reverse adjusters in that you pull the strap down to tighten, I believe with the Caterham/Luke ones you pull up (sure someone will confirm), best see which arrangement you prefer.
Also the Willans eyebolts do not allow the belts so swivel as you can't fit a tophat but again that doesn't seem to bother most people. Mine are bolted down tight and I don't have any hassle with them, don't even notice it.
I went for red Willans 3"shoulder / 2" lap 4 point, turnbuckle with the steel adjusters ☹️ and paid IIRC £220 for a pair.
Dave
ps was it you enquiring about the RM? Any thoughts?