Both my ps3 and 360 are 20gig. Also, both my 360 and ps3 have over 18 of their 20 gigs free if I remember correctly.
but I have an external 500gig on my desktop that the 360 can read from over the network. the 360 just has so many more codecs than the ps3 to play different file types.
the ps3 has to be plugged directly into the external HD, but can only recognize 20 or 30 files on the entire HD, most of them pictures
That, coupled with the netflix on the 360, the only thing I have on my HD are game files and gamer profiles.