While there’s no real rough stuff, I reckon being able to sit down and push a gear on the ‘cobbles’ on a FS would be more efficient than being on a HT that’ll skip and skitter over them.
Based on my experience on sections like the flat bits from USP to the boundary with Crowthorne and on to the green gate, where on a HT / SS I always end up feeling like I’ve had a jackhammer shoved up my arse.
(OK, what I *imagine* that’d feel like)
[edit – and 1:18 when I was younger, fitter and lighter, so my experience is probably irrelevant compared to a race whippet on a carbon HT who’d ride it all standing up]