Yes, it is better when thumb cluster is higher than the rest of the keys. That is because natural thumb position is higher than the tips of fingers. There was a lot of discussion here about whether it is better to press keys with thumbs using mostly a horizontal motion (e.g.
Oobly builds an ergoboard) or using mostly vertical motion (e.g. Ergodox or my
Katy). My opinion is that it does not matter much. 3D contoured keyboard is nice but not that important. Good thumb clusters and a split keyboard are the most important features.
The keyboards you listed look OK except the designer keyboard. It seems to use an wheel instead of thumb cluster keys. If so than it is a piece of crap.
I posted
my opinion on dactyl manuform.
Anyway, I do not think it is critical so that the thumb clusters are in a different plane. The critical thing is that you have a bunch of small(*) easily accessible keys very near to the natural/home location of thumbs. It is very comfortable to put all the shift modifiers on the thumb clusters. If the thumb clusters have some spare keys after all the modifiers are placed on them then add space and backspace and possibly more keys to the thumb clusters. E.g. here is
my preferred layout.
(*) Thumb cluster keys should be small so that you can place more of them as near as possible to the natural thumb position. It is easy to precisely hit them when they are near to the natural position.
