I tried both ST and HT when at the purchase stage and for the NC the PRHT was quieter on the motorway, provided it was closed properly with no leaking seals, and the drain flaps were properly closed. We looked at maybe twenty cars and test drove seven.
This lightweight car is never going to be as quiet as a saloon, which has extra many kg of strategically placed sound deadening and air-tight doors and windows.
With mine I found the old hard tyres were the loudest problem, changing them to supple grippy new ones made a big difference.
Drain flaps can be a problem, a twig was holding one of my NC flaps slightly open. I still can’t see how it got there past the filters unless someone was fiddling about underneath at some time past.
Standard exhaust was not a problem compared with the Bridgestone road noise.
GummiPfledge on the rubber roof seals and checking all the adjustments are correct helps a lot to reduce wind noise.
A bit of sound deadening in certain places in the boot also helps get rid of some tingy and drumming noises, but not all.