If this is to be your only mp3 player then the HD5 is the winner, better sound, larger capacity, still quite small (for a 20gb). Limiting yourself to these two isnt wise though, i like to be able to use my player at any PC, upload and download to any pc - Ipods and walkman are not good at that - 3rd party software for ipod, sonys as far as i know cant do that at all.
Once the WOW factor wears off (usually about a week for me) you want something that does what you need it to, i got rid of my Nano after 4 weeks but still have a HD5 for home use and a U10 for out and about.
If i was buying one player it would be the Iaudio X5.