Well cost you will just have to look at, the cost per/gb is prob going to be a bit lower on the 2tb drives. Expansion wise you can can obviously end up with a higher total amount if you use the 3Tb drives, and have a finite amount of drive spaces.
I just stick all of the drives in an old computer running linux in the basement. It's my 24/7 fileserver. Also because it's on all the time I tell it do things like run all my automated scripts, scan rss feeds for torrents so It can auto-download them the hour they are released.