I probbly ride my SS more than my geared bike, as it's less hassle. The SS is the bike I can just grab and go, whereas the geared seems to need checking over a lot more; it needs to be kept clean, too, which is a pain, during our muddy winters. I can just bash the dried mud off the SS, bung a bit of oil on't chain, and off I go.
Somewhere like Epping, the SS is perfect, as it's pretty flat. Swinley does not in any way have any major climbs, even the Wall is doable. As for the descents, well, they're the twisty turny type, so you're not actually getting up to big ring speed, really. More just let go of the brakes and steer. And the Labyrinthy bits have climby sections, so it's about being smooth, which I find the SS makes me concentrate more on.
Wales, and anywhere with proper hills, I'd takes the geared bike. I'm no masochist. Gears were invented for good reason.