The cosmos is lighter, comes with a 720p screen as standard and can have up to an 850m, where as the optimus is a bit heavier, has a 1080p screen as standard and can have a 765m (I assume this will soon be updated to an 860m or something).
I asked the very same question of PCS yesterday and I got this:
"The Cosmos only has 1 x HDD bay whereas the Optimus has 2 x HDD bay, with the possibility of an extra in the DVD bay, the Optimus is a bit more of a beast with its graphics capabilities. "
Likewise the new 8XX GPUs will be on the Optimus in around 2 weeks:
"No problem at all Mark, have just had a quick check, should be live in 2 weeks for the Optimus VI 15". "