I don't have any experience with those specifically, but I did use a folding keyboard for the purpose that you said (writing on the ipad), and it was an exercise in frustration. I also tried those folio type keyboards, and didn't like that either. I finally got the logitech keys to go, and it feels surprisingly good.
http://www.amazon.com/Logitech-Ultra-Portable-Bluetooth-Keyboard-920-006701/dp/B00R0I71S4
The clamcase also felt good, but it added too much weight for me.
Just as a note for the Thread Starter, the pictures of the "Logitech Keys-To-Go" might suggest that it is flexible or rubbery, it is not, it is indeed a solid keyboard made of hard plastic with a kind of smooth texture to it, it is really quite durable. That being said, if you might suggest getting the "Logitech Keys-To-Go", keep in mind that a HHKB is about the same size, perhaps even smaller, so you might as well just get a HHKB, but the "Logitech Keys-To-Go" will save you a bit of money.
Well.. it must be really durable. If it is as with all my previous logitech keyboards, I threw them away after 6-8 months. So in the long term, a HHKB may me more durable.
Also, on the HHKB you can replace caps but you cannot do that with this logitech thingy.
I've had mine for about 2 years, and it has gone with me in my bag almost every day. I'd think that replacing caps would be less of an issue than durability and size. And though the layout is the same size as the HHKB, the thickness is not. As the poster said, it is firm enough to give support, but it's only about as thick as a thin magazine.
Dimensions of HHKB from Amazon:
Item Weight 1.7 pounds
Product Dimensions 11.5 x 5 x 1.5 inches
Dimensions of Logitech Keys-to-Go from Amazon:
Item Weight 6.2 ounces
Product Dimensions 5.4 x 9.5 x 0.2 inches
As you can see, there's quite a difference in dimensions. Especially considering 1.7 pounds to 6.2 ounces (.3875 lbs) Or to put it another way, one weighs more than an iPad Air. The other one, considerably less.
