depends in what ur gonna use the hdd for. say, if ur just gonna store files or mp3, then get the 40gi 5200. however if ur gonna use this hdd for ur primary partition, ie c drive, then go for the 7200rpm one cause its helluva lot faster.
also getting a decent brand is advisable. ive got the imb 30gig 7200 delta. its going strong after 1 year and im sure its still got heaps of life left