If you want a back up HDD which will be kept on your desk, buy a 2TB GoFlex for 5200+.
If you want a portable HDD or one for your laptop or something to carry once in a while, go for a 1 TB Portable HDD as 1 TB is enough to have things while on the move for most people.
A 2.5" HDD is like 160gm or so while a 3.5" is like 500-1000gm or so.
Also, the 2.5" is like 1/4th or less the size of a 3.5", maybe 1/6th or so in fact.