Your better off going for the 120GB as it comes with a transfer cable to load all your data from the 20GB, with the 60GB you have to phone up Microsoft to send you one, it is free, but it takes a couple of weeks to get there.
the data transfer kit only comes with the new 120gb pack
the only other way is either emailing microsoft and trying to get them to send you one - but i think they only send if you upgrade consoles rather than just the HD