Or, you can do like I do, and have both. I have a Tungsten E (Palm) and a Toshiba e355 (Pocket PC). Bible reader on both and e-Sword on the Toshiba. Both take SD cards (Best Buy has 1GB for $20. I'm sure you can find cheaper, but these are SanDisk.)
I heartily second iSilo. I have it on both and have more reading than I can do in 10 years, including all the confessions, several systematic theologies, many philosophical works and miscellaneous light reading. iSiloX lets you convert internet files into something readable on the pocket devices. Also in the Toshiba I have Microsoft Reader with Calvin's Institutes, Owen's Death of Death, W.E.B.Du Bois' The Souls of Black Folk, Spurgeon's sermons, Shakespeare, Thoreau, Plato . . . you get the idea. Both machines can read Adobe files, but some files get mangled.
I don't know which one I would keep if I could only have one. Planetarium is only available for the Palm machines, and to me it is the best astronomy software out there. But, I like the ability of the Pocket PC to switch between tasks without closing other ones. Of course, we are talking Microsoft, so expect it to need re-booted occasionally (which I just did, again, this morning - twice.)
Don't have a wireless one, yet, but it is coming. I just need to decide which O.S. I like better and which one has the most usable software. Keep in mind that e-Sword is only available on the Pocket PC. But, BibleReader (which is free for the KJV), works on either and does a nice job.