Definitely hard drive for me. If I have games from a previous platform (like XBox, PS2, GC) then I also have that console. I'm not too worried about playing old games on the new platforms because I can always just plug in my old consoles. I don't think I've ever played an old game on a newer platform for that matter. Once I move on to the next generation I'm not interested in old platforms (unless new games are coming out for the, i.e. the PS2 but again I already own 2 PS2 systems).
But as time goes on the hard drive becomes more and more important. For one it allows me to play demos on my console w/o subscribing to a magazine and hoping they have the demo I want to play. This is a huge help for me because it is sometimes hard to gague how I will like a game just from the reviews. I have already dodged 3 games for the XBox 360 that I would have hated but that got good reviews. Also IIRC the PS3 has games that write a portion of themselves to the hard drive to cut down loading times (which is probably why there isn't an SKU w/o a hard drive). Plus I like to save my games to the console's hard drive instead of juggling memory cards.
