I still have the Oxford dictionary on my old Clie but in this case, I'm actually going to take a gamble on the Pocket PC version of PlecoDict when it comes out.
My main reason isn't related to the user-interface or OS, etc...actually, it's all about the battery.
I've always hated devices that have built-in, non-user replaceable batteries. No battery will last forever and it seems as if companies design these kinds of machines solely for the purpose of forcing you to upgrade whenever they cycle into a new product. The iPod is a great example. Replacing the battery is costly and takes some skill in soldering, not to mention jimmying open the whole case and scratching it. I think a battery compartment should be a basic requirement for any small electronic device.
The Palms always seem to come with built in batteries and that just bugs me. The Pocket PCs though can easily be opened and the batteries replaced. I've never been much concerned with matters of interface or anything, I just want a workhorse that's reliable and will last a long time, and the battery is a major component of that.