My Dahon is pretty comfortable (Speed 7), but I do have problems with riding flat bars for any distance, especially with small wheels. I don't like to do more than 10 miles on the folder.
I find the ride on the 20" wheels of the Dahon a lot better than the Brompton, but the Brommie does fold smaller. That said, however, I commute on busy London trains, and find the best method for getting it out of the way is to put it in the bike space, and fold the bars down, so it's pretty much flat against the wall.
On the subject of drop bars, I agree that they would be preferable, but spoil the fold. However, bullhorns could be folded to point in line with the stem and not interfere with the fold. I believe this is the case with the Speed TR.
Most comfortable folder IME is an Airnimal Chameleon, but it is an awkward fold. Smallest usable one is the Brompton. Best compromise (I think) is the Dahon.