For me, it would come down to size... I like small devices i can fit into a pocket so would pick the ipaq 4xxx series. I don't like integrated keyboards because i find using the stylus faster and if i need to do large amounts of data entry, i do it on a PC. Basically comes down to personal preference
They're all good devices
As for extended warranty: does it cover accidental breakage? IPAQ's are usually fairly reliable and the price you're quoting is a bit pricey though... so third party insurance might be better...