Gigabit network is gigabits per SECOND, not minute.
esata is the undisputed king of external speed (by a HUGE margin). But you have to deal with the incredible heat generated by the external devices. They can easily run into 55-60 degrees (idling) without proper cooling.
Depends on whether you need more portable storage or more space. Seems like if you don't need so much space, just get the Hitachi 180GB. The 2.5" usually use an extra USB cable for power instead of a separate power brick, one less item to carry around. And notebook hard drives are also usually more rugged since notebooks are designed to be moved around. They're usually slower though.
I have
1) 2 x 1TB RAID 1
2) 4 x 2TB RAID 10 (External controller + array + ups)
3) 2 x 250gb in RAID 0 (External controller + array)
4) 500gb + 80gb + 120gb
5) 80gb + 250 gb portable hdd
Usable:~ 6TB
Actual: 11TB
All usually connected to my main PC. Not counting my perpherial systems/notebooks
Think my recent 4x2TB was a bit of an overkill... But dumping hundreds of DVD backups is worth it.