3TB Time Capsule or a 3TB Hard Drive??

I already have an Airport Extream Wireless router and need a new larger hard drive for backups and Aperture work. Trying to decide on the 3TB Time Capsule or do I USB a new harddrive to the exsisting Airport. Or a new hard drive and put my current drive (400 GB) on the Airport USB to use for a network drive so it can be accessed with an iPad and Macbook Pro. Any thoughts??