Étape
Since the Étape involves mainly climbing, I’d go with a lightweight set of wheels any day. Even with my 13st bouncing around on them (my Xero XCR-1s do bend a bit when I’m out of the saddle), I reckon that’s more beneficial that heavier, but more aerodynamic wheels.
I’m a bit of a convert to handbuilts, and have a friend who has started his own wheelbuilding business. I shall be running a pair of his wheels (nothing fancy: open pros on 105 hubs) during the Tour of Flanders sportive. That’s if they survive the winter commuting….
With £600 you could get something pretty damned light and handbuilt from the likes of Pete Matthews or Paul Hewitt.