The two bar have different reaches. It's a question of what fits you properly (if at all). With Brompton it is not easy to change out the bar, although a slight riser bar can be subbed for the S bar (there' a company that produces a revision kit). But overall the decision should be based on proper form - does either bar put you in an optimum cycling position with your knees correctly places in relation to KOPS (for you, of course) and you weight balanced well with no excess pressure on your hands? Tires can be swapped if you decide you prefer Racers or Kojaks. But the bikes come with different stems depending on the bars so if you decide wrong, you have to not only replace the bars later but the stem riser as well (and cables). General consensus doesn't mean much, either, as it should be all about YOUR flexibility and comfort, not someone else's. Twitchiness goes away after you've ridden whatever small wheeler you buy for a while.