Oh, this might have had sth. to do with my feeling cramped. On the M-bar I tested (for a day and a half), I adjusted the saddle pretty much according to my road bike setup (almost KOPS, correct saddle height etc.). But I had the S-bar for much shorter and the saddle might have been too forward (I did adjust the saddle height but did not have time for more, as I had to return the bike to the shop quickly). So I guess I should definitely give the S-bar a second chance and make sure the pentaclip is in the correct position. Saddle height was definitely not an issue (as I've mentioned, my saddle height is only 71.5 cm, so I didn't even need to use the telescopic part of the seatpost that all these test bikes were equipped with), though I did have to stop occasionally and re-adjust the M-bar I tested due to slippage (these test bikes might not be maintained that well). But the S-bar then probably needs the pentaclip to face backward to allow for a more rearward saddle.
As for the twitchiness, this could have been exacerbated by the superlight construction and the tires. It's just that I was expecting the bike to be much less twitchy with the S-bar (with more body weight on the front wheel, that is a better weight distribution/balance on the bike itself)