Actually, I plan to keep both. I'll keep all my projects in Copy so I share them computer to computer, and since I already have a bunch of forum links to dropbox, I'll keep those live, and probably still use that folder for sharing that stuff.
You also get 2 gigs for sharing on Twitter, but referrals doesn't seem to have a max limit like dropbox does. (I'm sure that's temporary, like the 5 gigs is.)
Everyone should post a referral link, and future sign ups can choose one. (I'm feeling guilty now getting them all.) <img src="smileys/smiley9.gif" border="0" align="middle">
Edit: test link https://copy.com/PV9tlpCzNLsRzkjZ
Does make a scary looking link. (like a shortened URL)