The bigger disk you have the more you'll lose when it fails. ;)

The 1TB drives have 2 big problems ...
(1) they are all 3.5 inch drives ... 4 times the size and weight of the 2.5 inch drives
(2) they are not USB powered. They require an adaptor ... again adding a couple hundred grams and a lot of bulk.

The compact 2.5 inch USB powered drives are only available at the moment in 320 GB ... some in 500 GB

Dont buy anything that says its a "desktop" external hard disk. It means its big, bulky and requires and external power adapter.

USB pen drives are fine if you need relatively low capacity for storage ... as you say they are not that fast, ... and they are less durable than many people think - at theast the non-ruggedised ones are. If they get wet you could lose everything on them. The USB plug is subject to bending and breaking ... meaning your drive and data on it is useless until you can rebuild the USB male plug at the end of it. Since the USB pen drives usually stick out the side of your laptop when using them, they are surprisingly easy to damage. They just sit out there begging to be bent or broken. I have broken a couple in recent years.

You can now get a pretty rugged 64GB pen drive tho - so this is worth thinking about
Corsair 64GB USB2 Voyager Pen drive - Aria Technology

To avoid this problem it is possible to use short USB extension cable, this makes connection between drive and computer "flexible". However, nice compact cables are difficult to find.