There are no big differences between the different models as far as PocketStackz is concerned, Stackz will look fine on any Pocket PC. I have used it a lot myself on an Axim X3 (2004) and on a Fujitsu Siemens Pocket LOOX (2007). The former has a QVGA display, the latter VGA. VGA is nice especially for complicated kanji, but the lower QVGA resolution is completely sufficient as well, I even prefer this one because of the housing style.
It could be worth a look to check the handling of the arrow navigation buttons though. The study tools of PocketStackz are carefully designed to be conveniently controlled using one hand with the hardware buttons of the Pocket PC. This is a big advantage when learning on the go... of course you can also tap on the PocketStackz buttons with your fingers on the screen to avoid using the stylus, they are large enough for that purpose. But good Pocket PC hardware buttons offer a far better feel than touching the screen. And if the hardware buttons are used, the screen buttons can be hidden to reserve the entire display for the learning content.