Well, a complicated question because:
1, recent, non-low-end (everything above the Zire 21) palmOne devices have much better screens than QVGA Pocket PC's and are, in this respect, much better at reading e-books. Not only resolution-wise are they better than QVGA screens (even the standard HiRes Palm screens are 320*320, as opposed to the 320*240 QVGA PPC screens): they are just far less tiring on eyes, have far less blocks etc. Just put a Zire71 next to an iPAQ 2210 or 5450, and you will see a tremendous difference. The above-mentioned iPAQ screens flicker (this is a common problem with several iPAQ series), are blocky, and they render warm colors far worse than palmOne PDA's.
2, under Palm OS, it's possible to use much lower screen backlighting with third-party tools than on the Pocket PC. This is of extreme importance if you plan to read in the night.
Therefore, screen-wise, current palmOne devices offer much more than Pocket PC's in the same price range. Even a second-hand Zire 71 (you can get it under 100 US$, and, besides a great e-book device, it is a decent Palm device, runs Astraware games well etc.) will be much less tiring on your eyes than any QVGA PPC.
The above only applies to palmOne HiRes and HiRes+ devices versus QVGA PPC's
. VGA PPC's are a different question. Some of them are just excellent e-book readers; most notably, the iPAQ hx4700. Not just because it has a gorgeous, 4" screen that has even warmer (if a bit off) color reproduction (only the Pocket Loox 720 has great warm colors like the ones on the iPAQ screen), but also because its minimal brightness is really good for night-time reading, unlike, say, that of the Toshiba e830 or, to a lesser extent, the Pocket Loox 720. Therefore, an iPAQ hx4700 is indeed a great device for e-books screen/brightness-wise, even better than Palms.
The readers on the PPC are, however, much
1, First, there is absolutely no CHM support on the Palm OS platform, while on the PPC, there're quite a few CHM readers. (You may want to read my article at http://menneisyys.freeweb.hu/chm/
2, there is no µBook (http://www.gowerpoint.com/
) for the Palm. It's one of the best all-in-one readers for the PPC. For example, in some cases, it's even better at, say, rendering CHM files than all of the above-listed dedicated CHM readers.
3, as I've pointed out in my (beware, pretty old and unedited!) article on e-readers ) http://menneisyys.freeweb.hu/mwc/
), Palm OS doesn't support in-document images larger than 64 kbytes. There're e-book readers that make it possible to include >64k images in e-books, but, for example, iSilo and Mobipocket, the two most important readers, don't support this 'hack'. This is a problem if you want to read docs with a lot of high-resolution, large images with, say, Mobipocket (for example, tech docs with a lot of SVGA+ screenshots); in this case, the Palm platform will be a worse choice than the PPC.
4, there is no LIT reader for the Palm either. I don't think this is a big problem, though - after all, you can always extract the HTML files + the pictures even from your DRM-protected files and recomress them to a much more reader-friendly format (for example, Mobipocket).
However, if you don't plan to read CHM's and will never read tech docs that need large pics, Palm (particualrly palmOne) devices may prove better than QVGA PPCs because of their superior screens.
What would I personally choose?
If you have very little money and don't want to use your device for anything else than book reading (without support for CHM), get, say, a second-hand Palm T3 or Zire 71. Their screens are just unbeatable at this price segment.
If you have a lot of money and want a great all-in-one PDA that is also
excellent at reading, get an iPAQ hx4700 or, if you need the special features (USB host, CIR etc)/jog dial/more memory, the Pocket Loox 720 (beware, the latter's minimal brightness is considerably higher than that of the hx4700 and there're no known workarounds for this!). Much as I'm a PL720 owner, in this respect I think the hx4700 is much better.
Also see for example http://www.firstloox.org//forums/showthread.php?t=2875
on some of the related subjects. They only add tidbits to what I've said and will not give you a comprehensive "bird's view" picture, though.