Y580 is the same price
but doesn't have the 16gb SSD.
However, you'd want at least at least a 60gb SSD to make it useful anyway. I supposed you could get away with 32GB SSD if you just put OS + Browser on it.
Y500 is newer and has a better design/cooling. You could go SLI with y500 but that's a bit excessive and probably invites other issues as I don't recall multi-gpu solutions with ultrabay before this model so there are bound to be bugs.
EDIT: Ha, found the biggest flaw with the y500. Because of SLI capabilities, it doesn't support Optimus, hence no using integrated graphics. There goes the battery life... Also, many complain false advertisement of switchable graphics and WIDI (Intel integrated graphics thing) capabilities.