I have an old Jornada 545 and have been very pleased with it. Actually it is my second, the first my daughter nicked and absolutely adores!
I am impressed by the specs of the new Dell Axim, particularly as the Xscale technology it uses should be more 'future-proof' than some. I also like the fact that it has 2 types of expansion slot: compactflash and SD, so one can have a memory expansion card and a modem card (say) in use at the same time. But I'v enever used one, so all this is just from reading the reviews of it!
Really you need to decide which is more important, small size or expandability. As ever in computing it is a matter of working out what you are willing to compromise on!