To be honest, the three products are a bit too different to compare them directly.
Hitachi submitted a very convenient hard drive upgrade kit, which requires you to replace your notebook hard drive, but also allows you to keep using your existing drive in an external enclosure via USB 2.0. The Travelstar 2.5" 2-in.1 Upgrade Kit 160 GB comes with software that clones the entire contents of your existing hard drive, so you get a very convenient, all-in-one solution to upgrade your notebook hard drive.
Toshiba is new to the retail storage market and decided to take a rather conservative approach. The USB 2.0 Portable External Hard Drive 160 GB is solid and quiet and it comes with powerful backup software. It supports revisioning, so you can keep various file versions before a future backup process will overwrite outdated files.
While some may argue over its design, the features of Seagate's FreeAgent drive family are unmatched. The FreeAgent Go 160 GB does not offer traditional backup software, but its software suite is capable of syncing files between any host computer and the FreeAgent drive. This allows the user to even synchronize files between two or more computers. But the real bonus is the Ceedo software, which allows for the installation of compatible applications onto the FreeAgent drive, so you carry them with you everywhere you go, and you can execute them on multiple host computers. If you don't only want to take your data with you but want to keep your applications handy, the FreeAgent Go is for you.