Khufu vs. Duomid Headspace

Maybe not your cup of tea but I think both shelters have published dimensions on line and are of a simple straight line shape.
So go out to your nearest lawn with stakes , a long piece of string , hiking poles, some tape, a measuring tape and your mat and sleeping bag , build those two shelters and see how you and your kit fit inside.
Your perception and mine of the exact same space can be very different…
BTW, in those shelters the difference between sitting on a .5" mat (or none…) and a 2.5" mat can be rather dramatic.

The above photo is neither of those two shelter, just meant to illustrate the point…

To elaborate a bit on that, that photo shows the exact dimensions of an inner that some 6' something people say works for them.
Sleeping sideways in that particular inner is not an option.
(not "my" inner, the real one)
I am 5'8"…..

I believe you have it spot on when talking about pad thickness size when talking about shelters with slopped walls. I might have to try that field test.

Is that your hiking pole, or are you just happy to see us :O

I am compensating there…

I can't speak for the Duomid, but find with the Khufu that the mid-panel tie-outs are really effective in shaping the head space. As a solo user, I find the head space in the Khufu more than adequate, and the entire tarp fits me well, particularly with the use of two poles and the DPTE (I'm 5ft 8 1/2"). I really recommend the DPTE, though it is a little fiddly to set up. So far I have only used a relatively thin pad (Z-lite), not an inflatable.

There are a couple of more considerations that might be relevant.

Firstly, the desire for head room wants to be balanced by the need for structural stability in high wind, and a lower apex means less wind resistance.

Secondly, the apex wants to be within reasonable reach when you are supine – e.g. sometimes I have a tiny (eGear) lantern hanging from the apex (which perversely requires two hands to turn off and on – a real design flaw), which I want to be able to turn off and on without having to exert more effort than necessary. A more distant apex means more effort to reach it. Trivial perhaps, but that's tent life…

By the way, Franco's idea of making a mock-up is a good one, but requires a skin (fabric); you can get a completely unrealistic sense of the space from just the cord outline without infill. Something like people buying houses on the basis of viewing them empty; they change dramatically when filled with furniture, and suddenly space becomes at a premium. So I suggest you drape some cloth over the mockup…

I have the same Cuben Khufu as Robert above. I'm 184 (6,1") and haven't been able to use the Khufu a lot yet, but of course tried it a bit. With an Exped UL 7 it did work but remember worrying about it if I would have a big bag. I had a crap pitch on rocky ground though, and I think what Robert says about the mid tie outs helping out is right. The Khufu is very very well made, you can tell the proud Japanese craftsmanship. It is slightly lighter than a Duomid. I like that the Duomid has these little mitt clips inside the tarp for hanging a light or some rope from. Something the Khufu doesn't have. When I compared the two, it seemed the Khufu was ever so slightly bigger. I don't think you can go wrong with either of them though.